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rfc:rfc2683

Network Working Group B. Leiba Request for Comments: 2683 IBM T.J. Watson Research Center Category: Informational September 1999

                IMAP4 Implementation Recommendations

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (1999).  All Rights Reserved.

1. Abstract

 The IMAP4 specification [RFC-2060] describes a rich protocol for use
 in building clients and servers for storage, retrieval, and
 manipulation of electronic mail.  Because the protocol is so rich and
 has so many implementation choices, there are often trade-offs that
 must be made and issues that must be considered when designing such
 clients and servers.  This document attempts to outline these issues
 and to make recommendations in order to make the end products as
 interoperable as possible.

2. Conventions used in this document

 In examples, "C:" indicates lines sent by a client that is connected
 to a server.  "S:" indicates lines sent by the server to the client.
 The words "must", "must not", "should", "should not", and "may" are
 used with specific meaning in this document; since their meaning is
 somewhat different from that specified in RFC 2119, we do not put
 them in all caps here.  Their meaning is as follows:
 must --       This word means that the action described is necessary
               to ensure interoperability.  The recommendation should
               not be ignored.
 must not --   This phrase means that the action described will be
               almost certain to hurt interoperability.  The
               recommendation should not be ignored.

Leiba Informational [Page 1] RFC 2683 IMAP4 Implementation Recommendations September 1999

 should --     This word means that the action described is strongly
               recommended and will enhance interoperability or
               usability.  The recommendation should not be ignored
               without careful consideration.
 should not -- This phrase means that the action described is strongly
               recommended against, and might hurt interoperability or
               usability.  The recommendation should not be ignored
               without careful consideration.
 may --        This word means that the action described is an
               acceptable implementation choice.  No specific
               recommendation is implied; this word is used to point
               out a choice that might not be obvious, or to let
               implementors know what choices have been made by
               existing implementations.

3. Interoperability Issues and Recommendations

3.1. Accessibility

 This section describes the issues related to access to servers and
 server resources.  Concerns here include data sharing and maintenance
 of client/server connections.

3.1.1. Multiple Accesses of the Same Mailbox

 One strong point of IMAP4 is that, unlike POP3, it allows for
 multiple simultaneous access to a single mailbox.  A user can, thus,
 read mail from a client at home while the client in the office is
 still connected; or the help desk staff can all work out of the same
 inbox, all seeing the same pool of questions.  An important point
 about this capability, though is that NO SERVER IS GUARANTEED TO
 SUPPORT THIS.  If you are selecting an IMAP server and this facility
 is important to you, be sure that the server you choose to install,
 in the configuration you choose to use, supports it.
 If you are designing a client, you must not assume that you can
 access the same mailbox more than once at a time.  That means
 1. you must handle gracefully the failure of a SELECT command if the
    server refuses the second SELECT,
 2. you must handle reasonably the severing of your connection (see
    "Severed Connections", below) if the server chooses to allow the
    second SELECT by forcing the first off,
 3. you must avoid making multiple connections to the same mailbox in
    your own client (for load balancing or other such reasons), and
 4. you must avoid using the STATUS command on a mailbox that you have
    selected (with some server implementations the STATUS command has
    the same problems with multiple access as do the SELECT and

Leiba Informational [Page 2] RFC 2683 IMAP4 Implementation Recommendations September 1999

    EXAMINE commands).
 A further note about STATUS: The STATUS command is sometimes used to
 check a non-selected mailbox for new mail.  This mechanism must not
 be used to check for new mail in the selected mailbox; section 5.2 of
 [RFC-2060] specifically forbids this in its last paragraph.  Further,
 since STATUS takes a mailbox name it is an independent operation, not
 operating on the selected mailbox.  Because of this, the information
 it returns is not necessarily in synchronization with the selected
 mailbox state.

3.1.2. Severed Connections

 The client/server connection may be severed for one of three reasons:
 the client severs the connection, the server severs the connection,
 or the connection is severed by outside forces beyond the control of
 the client and the server (a telephone line drops, for example).
 Clients and servers must both deal with these situations.
 When the client wants to sever a connection, it's usually because it
 has finished the work it needed to do on that connection.  The client
 should send a LOGOUT command, wait for the tagged response, and then
 close the socket.  But note that, while this is what's intended in
 the protocol design, there isn't universal agreement here.  Some
 contend that sending the LOGOUT and waiting for the two responses
 (untagged BYE and tagged OK) is wasteful and unnecessary, and that
 the client can simply close the socket.  The server should interpret
 the closed socket as a log out by the client.  The counterargument is
 that it's useful from the standpoint of cleanup, problem
 determination, and the like, to have an explicit client log out,
 because otherwise there is no way for the server to tell the
 difference between "closed socket because of log out" and "closed
 socket because communication was disrupted".  If there is a
 client/server interaction problem, a client which routinely
 terminates a session by breaking the connection without a LOGOUT will
 make it much more difficult to determine the problem.
 Because of this disagreement, server designers must be aware that
 some clients might close the socket without sending a LOGOUT.  In any
 case, whether or not a LOGOUT was sent, the server should not
 implicitly expunge any messages from the selected mailbox.  If a
 client wants the server to do so, it must send a CLOSE or EXPUNGE
 command explicitly.
 When the server wants to sever a connection it's usually due to an
 inactivity timeout or is because a situation has arisen that has
 changed the state of the mail store in a way that the server can not
 communicate to the client.  The server should send an untagged BYE

Leiba Informational [Page 3] RFC 2683 IMAP4 Implementation Recommendations September 1999

 response to the client and then close the socket.  Sending an
 untagged BYE response before severing allows the server to send a
 human-readable explanation of the problem to the client, which the
 client may then log, display to the user, or both (see section 7.1.5
 of [RFC-2060]).
 Regarding inactivity timeouts, there is some controversy.  Unlike
 POP, for which the design is for a client to connect, retrieve mail,
 and log out, IMAP's design encourages long-lived (and mostly
 inactive) client/server sessions.  As the number of users grows, this
 can use up a lot of server resources, especially with clients that
 are designed to maintain sessions for mailboxes that the user has
 finished accessing.  To alleviate this, a server may implement an
 inactivity timeout, unilaterally closing a session (after first
 sending an untagged BYE, as noted above).  Some server operators have
 reported dramatic improvements in server performance after doing
 this.  As specified in [RFC-2060], if such a timeout is done it must
 not be until at least 30 minutes of inactivity.  The reason for this
 specification is to prevent clients from sending commands (such as
 NOOP) to the server at frequent intervals simply to avert a too-early
 timeout.  If the client knows that the server may not time out the
 session for at least 30 minutes, then the client need not poll at
 intervals more frequent than, say, 25 minutes.

3.2. Scaling

 IMAP4 has many features that allow for scalability, as mail stores
 become larger and more numerous.  Large numbers of users, mailboxes,
 and messages, and very large messages require thought to handle
 efficiently.  This document will not address the administrative
 issues involved in large numbers of users, but we will look at the
 other items.

3.2.1. Flood Control

 There are three situations when a client can make a request that will
 result in a very large response - too large for the client reasonably
 to deal with: there are a great many mailboxes available, there are a
 great many messages in the selected mailbox, or there is a very large
 message part.  The danger here is that the end user will be stuck
 waiting while the server sends (and the client processes) an enormous
 response.  In all of these cases there are things a client can do to
 reduce that danger.
 There is also the case where a client can flood a server, by sending
 an arbitratily long command.  We'll discuss that issue, too, in this
 section.

Leiba Informational [Page 4] RFC 2683 IMAP4 Implementation Recommendations September 1999

3.2.1.1. Listing Mailboxes

 Some servers present Usenet newsgroups to IMAP users.  Newsgroups,
 and other such hierarchical mailbox structures, can be very numerous
 but may have only a few entries at the top level of hierarchy.  Also,
 some servers are built against mail stores that can, unbeknownst to
 the server, have circular hierarchies - that is, it's possible for
 "a/b/c/d" to resolve to the same file structure as "a", which would
 then mean that "a/b/c/d/b" is the same as "a/b", and the hierarchy
 will never end.  The LIST response in this case will be unlimited.
 Clients that will have trouble with this are those that use
     C: 001 LIST "" *
 to determine the mailbox list.  Because of this, clients should not
 use an unqualified "*" that way in the LIST command.  A safer
 approach is to list each level of hierarchy individually, allowing
 the user to traverse the tree one limb at a time, thus:
     C: 001 LIST "" %
     S: * LIST () "/" Banana
     S: * LIST ...etc...
     S: 001 OK done
 and then
     C: 002 LIST "" Banana/%
     S: * LIST () "/" Banana/Apple
     S: * LIST ...etc...
     S: 002 OK done
 Using this technique the client's user interface can give the user
 full flexibility without choking on the voluminous reply to "LIST *".
 Of course, it is still possible that the reply to
     C: 005 LIST "" alt.fan.celebrity.%
 may be thousands of entries long, and there is, unfortunately,
 nothing the client can do to protect itself from that.  This has not
 yet been a notable problem.
 Servers that may export circular hierarchies (any server that
 directly presents a UNIX file system, for instance) should limit the
 hierarchy depth to prevent unlimited LIST responses.  A suggested
 depth limit is 20 hierarchy levels.

Leiba Informational [Page 5] RFC 2683 IMAP4 Implementation Recommendations September 1999

3.2.1.2. Fetching the List of Messages

 When a client selects a mailbox, it is given a count, in the untagged
 EXISTS response, of the messages in the mailbox.  This number can be
 very large.  In such a case it might be unwise to use
     C: 004 FETCH 1:* ALL
 to populate the user's view of the mailbox.  One good method to avoid
 problems with this is to batch the requests, thus:
     C: 004 FETCH 1:50 ALL
     S: * 1 FETCH ...etc...
     S: 004 OK done
     C: 005 FETCH 51:100 ALL
     S: * 51 FETCH ...etc...
     S: 005 OK done
     C: 006 FETCH 101:150 ALL
     ...etc...
 Using this method, another command, such as "FETCH 6 BODY[1]" can be
 inserted as necessary, and the client will not have its access to the
 server blocked by a storm of FETCH replies.  (Such a method could be
 reversed to fetch the LAST 50 messages first, then the 50 prior to
 that, and so on.)
 As a smart extension of this, a well designed client, prepared for
 very large mailboxes, will not automatically fetch data for all
 messages AT ALL.  Rather, the client will populate the user's view
 only as the user sees it, possibly pre-fetching selected information,
 and only fetching other information as the user scrolls to it.  For
 example, to select only those messages beginning with the first
 unseen one:
     C: 003 SELECT INBOX
     S: * 10000 EXISTS
     S: * 80 RECENT
     S: * FLAGS (\Answered \Flagged \Deleted \Draft \Seen)
     S: * OK [UIDVALIDITY 824708485] UID validity status
     S: * OK [UNSEEN 9921] First unseen message
     S: 003 OK [READ-WRITE] SELECT completed
     C: 004 FETCH 9921:* ALL
     ... etc...
 If the server does not return an OK [UNSEEN] response, the client may
 use SEARCH UNSEEN to obtain that value.

Leiba Informational [Page 6] RFC 2683 IMAP4 Implementation Recommendations September 1999

 This mechanism is good as a default presentation method, but only
 works well if the default message order is acceptable.  A client may
 want to present various sort orders to the user (by subject, by date
 sent, by sender, and so on) and in that case (lacking a SORT
 extension on the server side) the client WILL have to retrieve all
 message descriptors.  A client that provides this service should not
 do it by default and should inform the user of the costs of choosing
 this option for large mailboxes.

3.2.1.3. Fetching a Large Body Part

 The issue here is similar to the one for a list of messages.  In the
 BODYSTRUCTURE response the client knows the size, in bytes, of the
 body part it plans to fetch.  Suppose this is a 70 MB video clip. The
 client can use partial fetches to retrieve the body part in pieces,
 avoiding the problem of an uninterruptible 70 MB literal coming back
 from the server:
     C: 022 FETCH 3 BODY[1]<0.20000>
     S: * 3 FETCH (FLAGS(\Seen) BODY[1]<0> {20000}
     S: ...data...)
     S: 022 OK done
     C: 023 FETCH 3 BODY[1]<20001.20000>
     S: * 3 FETCH (BODY[1]<20001> {20000}
     S: ...data...)
     S: 023 OK done
     C: 024 FETCH 3 BODY[1]<40001.20000>
     ...etc...

3.2.1.4. BODYSTRUCTURE vs. Entire Messages

 Because FETCH BODYSTRUCTURE is necessary in order to determine the
 number of body parts, and, thus, whether a message has "attachments",
 clients often use FETCH FULL as their normal method of populating the
 user's view of a mailbox.  The benefit is that the client can display
 a paperclip icon or some such indication along with the normal
 message summary.  However, this comes at a significant cost with some
 server configurations.  The parsing needed to generate the FETCH
 BODYSTRUCTURE response may be time-consuming compared with that
 needed for FETCH ENVELOPE.  The client developer should consider this
 issue when deciding whether the ability to add a paperclip icon is
 worth the tradeoff in performance, especially with large mailboxes.
 Some clients, rather than using FETCH BODYSTRUCTURE, use FETCH BODY[]
 (or the equivalent FETCH RFC822) to retrieve the entire message.
 They then do the MIME parsing in the client.  This may give the
 client slightly more flexibility in some areas (access, for instance,
 to header fields that aren't returned in the BODYSTRUCTURE and

Leiba Informational [Page 7] RFC 2683 IMAP4 Implementation Recommendations September 1999

 ENVELOPE responses), but it can cause severe performance problems by
 forcing the transfer of all body parts when the user might only want
 to see some of them - a user logged on by modem and reading a small
 text message with a large ZIP file attached may prefer to read the
 text only and save the ZIP file for later.  Therefore, a client
 should not normally retrieve entire messages and should retrieve
 message body parts selectively.

3.2.1.5. Long Command Lines

 A client can wind up building a very long command line in an effort to
 try to be efficient about requesting information from a server.  This
 can typically happen when a client builds a message set from selected
 messages and doesn't recognise that contiguous blocks of messages may
 be group in a range.  Suppose a user selects all 10,000 messages in a
 large mailbox and then unselects message 287.  The client could build
 that message set as "1:286,288:10000", but a client that doesn't
 handle that might try to enumerate each message individually and build
 "1,2,3,4, [and so on] ,9999,10000".  Adding that to the fetch command
 results in a command line that's almost 49,000 octets long, and,
 clearly, one can construct a command line that's even longer.
 A client should limit the length of the command lines it generates to
 approximately 1000 octets (including all quoted strings but not
 including literals).  If the client is unable to group things into
 ranges so that the command line is within that length, it should
 split the request into multiple commands.  The client should use
 literals instead of long quoted strings, in order to keep the command
 length down.
 For its part, a server should allow for a command line of at least
 8000 octets.  This provides plenty of leeway for accepting reasonable
 length commands from clients.  The server should send a BAD response
 to a command that does not end within the server's maximum accepted
 command length.

3.2.2. Subscriptions

 The client isn't the only entity that can get flooded: the end user,
 too, may need some flood control.  The IMAP4 protocol provides such
 control in the form of subscriptions.  Most servers support the
 SUBSCRIBE, UNSUBSCRIBE, and LSUB commands, and many users choose to
 narrow down a large list of available mailboxes by subscribing to the
 ones that they usually want to see.  Clients, with this in mind,
 should give the user a way to see only subscribed mailboxes.  A
 client that never uses the LSUB command takes a significant usability
 feature away from the user.  Of course, the client would not want to
 hide the LIST command completely; the user needs to have a way to

Leiba Informational [Page 8] RFC 2683 IMAP4 Implementation Recommendations September 1999

 choose between LIST and LSUB.  The usual way to do this is to provide
 a setting like "show which mailboxes?:  [] all  [] subscribed only".

3.2.3. Searching

 IMAP SEARCH commands can become particularly troublesome (that is,
 slow) on mailboxes containing a large number of messages.  So let's
 put a few things in perspective in that regard.
 The flag searches should be fast.  The flag searches (ALL, [UN]SEEN,
 [UN]ANSWERED, [UN]DELETED, [UN]DRAFT, [UN]FLAGGED, NEW, OLD, RECENT)
 are known to be used by clients for the client's own use (for
 instance, some clients use "SEARCH UNSEEN" to find unseen mail and
 "SEARCH DELETED" to warn the user before expunging messages).
 Other searches, particularly the text searches (HEADER, TEXT, BODY)
 are initiated by the user, rather than by the client itself, and
 somewhat slower performance can be tolerated, since the user is aware
 that the search is being done (and is probably aware that it might be
 time-consuming).  A smart server might use dynamic indexing to speed
 commonly used text searches.
 The client may allow other commands to be sent to the server while a
 SEARCH is in progress, but at the time of this writing there is
 little or no server support for parallel processing of multiple
 commands in the same session (and see "Multiple Accesses of the Same
 Mailbox" above for a description of the dangers of trying to work
 around this by doing your SEARCH in another session).
 Another word about text searches: some servers, built on database
 back-ends with indexed search capabilities, may return search results
 that do not match the IMAP spec's "case-insensitive substring"
 requirements.  While these servers are in violation of the protocol,
 there is little harm in the violation as long as the search results
 are used only in response to a user's request.  Still, developers of
 such servers should be aware that they ARE violating the protocol,
 should think carefully about that behaviour, and must be certain that
 their servers respond accurately to the flag searches for the reasons
 outlined above.
 In addition, servers should support CHARSET UTF-8 [UTF-8] in
 searches.

Leiba Informational [Page 9] RFC 2683 IMAP4 Implementation Recommendations September 1999

3.3 Avoiding Invalid Requests

 IMAP4 provides ways for a server to tell a client in advance what is
 and isn't permitted in some circumstances.  Clients should use these
 features to avoid sending requests that a well designed client would
 know to be invalid.  This section explains this in more detail.

3.3.1. The CAPABILITY Command

 All IMAP4 clients should use the CAPABILITY command to determine what
 version of IMAP and what optional features a server supports.  The
 client should not send IMAP4rev1 commands and arguments to a server
 that does not advertize IMAP4rev1 in its CAPABILITY response.
 Similarly, the client should not send IMAP4 commands that no longer
 exist in IMAP4rev1 to a server that does not advertize IMAP4 in its
 CAPABILITY response.  An IMAP4rev1 server is NOT required to support
 obsolete IMAP4 or IMAP2bis commands (though some do; do not let this
 fact lull you into thinking that it's valid to send such commands to
 an IMAP4rev1 server).
 A client should not send commands to probe for the existance of
 certain extensions.  All standard and standards-track extensions
 include CAPABILITY tokens indicating their presense.  All private and
 experimental extensions should do the same, and clients that take
 advantage of them should use the CAPABILITY response to determine
 whether they may be used or not.

3.3.2. Don't Do What the Server Says You Can't

 In many cases, the server, in response to a command, will tell the
 client something about what can and can't be done with a particular
 mailbox.  The client should pay attention to this information and
 should not try to do things that it's been told it can't do.
 Examples:
  • Do not try to SELECT a mailbox that has the \Noselect flag set.
  • Do not try to CREATE a sub-mailbox in a mailbox that has the

\Noinferiors flag set.

  • Do not respond to a failing COPY or APPEND command by trying to

CREATE the target mailbox if the server does not respond with a

    [TRYCREATE] response code.
 *  Do not try to expunge a mailbox that has been selected with the
    [READ-ONLY] response code.

Leiba Informational [Page 10] RFC 2683 IMAP4 Implementation Recommendations September 1999

3.4. Miscellaneous Protocol Considerations

 We describe here a number of important protocol-related issues, the
 misunderstanding of which has caused significant interoperability
 problems in IMAP4 implementations.  One general item is that every
 implementer should be certain to take note of and to understand
 section 2.2.2 and the preamble to section 7 of the IMAP4rev1 spec
 [RFC-2060].

3.4.1. Well Formed Protocol

 We cannot stress enough the importance of adhering strictly to the
 protocol grammar.  The specification of the protocol is quite rigid;
 do not assume that you can insert blank space for "readability" if
 none is called for.  Keep in mind that there are parsers out there
 that will crash if there are protocol errors.  There are clients that
 will report every parser burp to the user.  And in any case,
 information that cannot be parsed is information that is lost.  Be
 careful in your protocol generation.  And see "A Word About Testing",
 below.
 In particular, note that the string in the INTERNALDATE response is
 NOT an RFC-822 date string - that is, it is not in the same format as
 the first string in the ENVELOPE response.  Since most clients will,
 in fact, accept an RFC-822 date string in the INTERNALDATE response,
 it's easy to miss this in your interoperability testing.  But it will
 cause a problem with some client, so be sure to generate the correct
 string for this field.

3.4.2. Special Characters

 Certain characters, currently the double-quote and the backslash, may
 not be sent as-is inside a quoted string.  These characters must be
 preceded by the escape character if they are in a quoted string, or
 else the string must be sent as a literal.  Both clients and servers
 must handle this, both on output (they must send these characters
 properly) and on input (they must be able to receive escaped
 characters in quoted strings).  Example:
     C: 001 LIST "" %
     S: * LIST () "" INBOX
     S: * LIST () "\\" TEST
     S: * LIST () "\\" {12}
     S: "My" mailbox
     S: 001 OK done
     C: 002 LIST "" "\"My\" mailbox\\%"
     S: * LIST () "\\" {17}
     S: "My" mailbox\Junk

Leiba Informational [Page 11] RFC 2683 IMAP4 Implementation Recommendations September 1999

     S: 002 OK done
 Note that in the example the server sent the hierarchy delimiter as
 an escaped character in the quoted string and sent the mailbox name
 containing imbedded double-quotes as a literal.  The client used only
 quoted strings, escaping both the backslash and the double-quote
 characters.
 The CR and LF characters may be sent ONLY in literals; they are not
 allowed, even if escaped, inside quoted strings.
 And while we're talking about special characters: the IMAP spec, in
 the section titled "Mailbox International Naming Convention",
 describes how to encode mailbox names in modified UTF-7 [UTF-7 and
 RFC-2060].  Implementations must adhere to this in order to be
 interoperable in the international market, and servers should
 validate mailbox names sent by client and reject names that do not
 conform.
 As to special characters in userids and passwords: clients must not
 restrict what a user may type in for a userid or a password.  The
 formal grammar specifies that these are "astrings", and an astring
 can be a literal.  A literal, in turn can contain any 8-bit
 character, and clients must allow users to enter all 8-bit characters
 here, and must pass them, unchanged, to the server (being careful to
 send them as literals when necessary).  In particular, some server
 configurations use "@" in user names, and some clients do not allow
 that character to be entered; this creates a severe interoperability
 problem.

3.4.3. UIDs and UIDVALIDITY

 Servers that support existing back-end mail stores often have no good
 place to save UIDs for messages.  Often the existing mail store will
 not have the concept of UIDs in the sense that IMAP has: strictly
 increasing, never re-issued, 32-bit integers.  Some servers solve
 this by storing the UIDs in a place that's accessible to end users,
 allowing for the possibility that the users will delete them.  Others
 solve it by re-assigning UIDs every time a mailbox is selected.
 The server should maintain UIDs permanently for all messages if it
 can.  If that's not possible, the server must change the UIDVALIDITY
 value for the mailbox whenever any of the UIDs may have become
 invalid.  Clients must recognize that the UIDVALIDITY has changed and
 must respond to that condition by throwing away any information that
 they have saved about UIDs in that mailbox.  There have been many
 problems in this area when clients have failed to do this; in the
 worst case it will result in loss of mail when a client deletes the

Leiba Informational [Page 12] RFC 2683 IMAP4 Implementation Recommendations September 1999

 wrong piece of mail by using a stale UID.
 It seems to be a common misunderstanding that "the UIDVALIDITY and
 the UID, taken together, form a 64-bit identifier that uniquely
 identifies a message on a server".  This is absolutely NOT TRUE.
 There is no assurance that the UIDVALIDITY values of two mailboxes be
 different, so the UIDVALIDITY in no way identifies a mailbox.  The
 ONLY purpose of UIDVALIDITY is, as its name indicates, to give the
 client a way to check the validity of the UIDs it has cached.  While
 it is a valid implementation choice to put these values together to
 make a 64-bit identifier for the message, the important concept here
 is that UIDs are not unique between mailboxes; they are only unique
 WITHIN a given mailbox.
 Some server implementations have attempted to make UIDs unique across
 the entire server.  This is inadvisable, in that it limits the life
 of UIDs unnecessarily.  The UID is a 32-bit number and will run out
 in reasonably finite time if it's global across the server.  If you
 assign UIDs sequentially in one mailbox, you will not have to start
 re-using them until you have had, at one time or another, 2**32
 different messages in that mailbox.  In the global case, you will
 have to reuse them once you have had, at one time or another, 2**32
 different messages in the entire mail store.  Suppose your server has
 around 8000 users registered (2**13).  That gives an average of 2**19
 UIDs per user.  Suppose each user gets 32 messages (2**5) per day.
 That gives you 2**14 days (16000+ days = about 45 years) before you
 run out.  That may seem like enough, but multiply the usage just a
 little (a lot of spam, a lot of mailing list subscriptions, more
 users) and you limit yourself too much.
 What's worse is that if you have to wrap the UIDs, and, thus, you
 have to change UIDVALIDITY and invalidate the UIDs in the mailbox,
 you have to do it for EVERY mailbox in the system, since they all
 share the same UID pool.  If you assign UIDs per mailbox and you have
 a problem, you only have to kill the UIDs for that one mailbox.
 Under extreme circumstances (and this is extreme, indeed), the server
 may have to invalidate UIDs while a mailbox is in use by a client -
 that is, the UIDs that the client knows about in its active mailbox
 are no longer valid.  In that case, the server must immediately
 change the UIDVALIDITY and must communicate this to the client.  The
 server may do this by sending an unsolicited UIDVALIDITY message, in
 the same form as in response to the SELECT command.  Clients must be
 prepared to handle such a message and the possibly coincident failure
 of the command in process.  For example:

Leiba Informational [Page 13] RFC 2683 IMAP4 Implementation Recommendations September 1999

     C: 032 UID STORE 382 +Flags.silent \Deleted
     S: * OK [UIDVALIDITY 12345] New UIDVALIDITY value!
     S: 032 NO UID command rejected because UIDVALIDITY changed!
     C: ...invalidates local information and re-fetches...
     C: 033 FETCH 1:* UID
     ...etc...
 At the time of the writing of this document, the only server known to
 do this does so only under the following condition: the client
 selects INBOX, but there is not yet a physical INBOX file created.
 Nonetheless, the SELECT succeeds, exporting an empty INBOX with a
 temporary UIDVALIDITY of 1.  While the INBOX remains selected, mail
 is delivered to the user, which creates the real INBOX file and
 assigns a permanent UIDVALIDITY (that is likely not to be 1).  The
 server reports the change of UIDVALIDITY, but as there were no
 messages before, so no UIDs have actually changed, all the client
 must do is accept the change in UIDVALIDITY.
 Alternatively, a server may force the client to re-select the
 mailbox, at which time it will obtain a new UIDVALIDITY value.  To do
 this, the server closes this client session (see "Severed
 Connections" above) and the client then reconnects and gets back in
 synch.  Clients must be prepared for either of these behaviours.
 We do not know of, nor do we anticipate the future existance of, a
 server that changes UIDVALIDITY while there are existing messages,
 but clients must be prepared to handle this eventuality.

3.4.4. FETCH Responses

 When a client asks for certain information in a FETCH command, the
 server may return the requested information in any order, not
 necessarily in the order that it was requested.  Further, the server
 may return the information in separate FETCH responses and may also
 return information that was not explicitly requested (to reflect to
 the client changes in the state of the subject message).  Some
 examples:
     C: 001 FETCH 1 UID FLAGS INTERNALDATE
     S: * 5 FETCH (FLAGS (\Deleted))
     S: * 1 FETCH (FLAGS (\Seen) INTERNALDATE "..." UID 345)
     S: 001 OK done
 (In this case, the responses are in a different order.  Also, the
 server returned a flag update for message 5, which wasn't part of the
 client's request.)

Leiba Informational [Page 14] RFC 2683 IMAP4 Implementation Recommendations September 1999

     C: 002 FETCH 2 UID FLAGS INTERNALDATE
     S: * 2 FETCH (INTERNALDATE "...")
     S: * 2 FETCH (UID 399)
     S: * 2 FETCH (FLAGS ())
     S: 002 OK done
 (In this case, the responses are in a different order and were
 returned in separate responses.)
     C: 003 FETCH 2 BODY[1]
     S: * 2 FETCH (FLAGS (\Seen) BODY[1] {14}
     S: Hello world!
     S: )
     S: 003 OK done
 (In this case, the FLAGS response was added by the server, since
 fetching the body part caused the server to set the \Seen flag.)
 Because of this characteristic a client must be ready to receive any
 FETCH response at any time and should use that information to update
 its local information about the message to which the FETCH response
 refers.  A client must not assume that any FETCH responses will come
 in any particular order, or even that any will come at all.  If after
 receiving the tagged response for a FETCH command the client finds
 that it did not get all of the information requested, the client
 should send a NOOP command to the server to ensure that the server
 has an opportunity to send any pending EXPUNGE responses to the
 client (see [RFC-2180]).

3.4.5. RFC822.SIZE

 Some back-end mail stores keep the mail in a canonical form, rather
 than retaining the original MIME format of the messages.  This means
 that the server must reassemble the message to produce a MIME stream
 when a client does a fetch such as RFC822 or BODY[], requesting the
 entire message.  It also may mean that the server has no convenient
 way to know the RFC822.SIZE of the message.  Often, such a server
 will actually have to build the MIME stream to compute the size, only
 to throw the stream away and report the size to the client.
 When this is the case, some servers have chosen to estimate the size,
 rather than to compute it precisely.  Such an estimate allows the
 client to display an approximate size to the user and to use the
 estimate in flood control considerations (q.v.), but requires that
 the client not use the size for things such as allocation of buffers,
 because those buffers might then be too small to hold the actual MIME
 stream.  Instead, a client should use the size that's returned in the
 literal when you fetch the data.

Leiba Informational [Page 15] RFC 2683 IMAP4 Implementation Recommendations September 1999

 The protocol requires that the RFC822.SIZE value returned by the
 server be EXACT.  Estimating the size is a protocol violation, and
 server designers must be aware that, despite the performance savings
 they might realize in using an estimate, this practice will cause
 some clients to fail in various ways.  If possible, the server should
 compute the RFC822.SIZE for a particular message once, and then save
 it for later retrieval.  If that's not possible, the server must
 compute the value exactly every time.  Incorrect estimates do cause
 severe interoperability problems with some clients.

3.4.6. Expunged Messages

 If the server allows multiple connections to the same mailbox, it is
 often possible for messages to be expunged in one client unbeknownst
 to another client.  Since the server is not allowed to tell the
 client about these expunged messages in response to a FETCH command,
 the server may have to deal with the issue of how to return
 information about an expunged message.  There was extensive
 discussion about this issue, and the results of that discussion are
 summarized in [RFC-2180].  See that reference for a detailed
 explanation and for recommendations.

3.4.7. The Namespace Issue

 Namespaces are a very muddy area in IMAP4 implementation right now
 (see [NAMESPACE] for a proposal to clear the water a bit).  Until the
 issue is resolved, the important thing for client developers to
 understand is that some servers provide access through IMAP to more
 than just the user's personal mailboxes, and, in fact, the user's
 personal mailboxes may be "hidden" somewhere in the user's default
 hierarchy.  The client, therefore, should provide a setting wherein
 the user can specify a prefix to be used when accessing mailboxes. If
 the user's mailboxes are all in "~/mail/", for instance, then the
 user can put that string in the prefix.  The client would then put
 the prefix in front of any name pattern in the LIST and LSUB
 commands:
     C: 001 LIST "" ~/mail/%
 (See also "Reference Names in the LIST Command" below.)

3.4.8. Creating Special-Use Mailboxes

 It may seem at first that this is part of the namespace issue; it is
 not, and is only indirectly related to it.  A number of clients like
 to create special-use mailboxes with particular names.  Most
 commonly, clients with a "trash folder" model of message deletion
 want to create a mailbox with the name "Trash" or "Deleted".  Some

Leiba Informational [Page 16] RFC 2683 IMAP4 Implementation Recommendations September 1999

 clients want to create a "Drafts" mailbox, an "Outbox" mailbox, or a
 "Sent Mail" mailbox.  And so on.  There are two major
 interoperability problems with this practice:
 1. different clients may use different names for mailboxes with
    similar functions (such as "Trash" and "Deleted"), or may manage
    the same mailboxes in different ways, causing problems if a user
    switches between clients and
 2. there is no guarantee that the server will allow the creation of
    the desired mailbox.
 The client developer is, therefore, well advised to consider
 carefully the creation of any special-use mailboxes on the server,
 and, further, the client must not require such mailbox creation -
 that is, if you do decide to do this, you must handle gracefully the
 failure of the CREATE command and behave reasonably when your
 special-use mailboxes do not exist and can not be created.
 In addition, the client developer should provide a convenient way for
 the user to select the names for any special-use mailboxes, allowing
 the user to make these names the same in all clients used and to put
 them where the user wants them.

3.4.9. Reference Names in the LIST Command

 Many implementers of both clients and servers are confused by the
 "reference name" on the LIST command.  The reference name is intended
 to be used in much the way a "cd" (change directory) command is used
 on Unix, PC DOS, Windows, and OS/2 systems.  That is, the mailbox
 name is interpreted in much the same way as a file of that name would
 be found if one had done a "cd" command into the directory specified
 by the reference name.  For example, in Unix we have the following:
     > cd /u/jones/junk
     > vi banana        [file is "/u/jones/junk/banana"]
     > vi stuff/banana  [file is "/u/jones/junk/stuff/banana"]
     > vi /etc/hosts    [file is "/etc/hosts"]
 In the past, there have been several interoperability problems with
 this.  First, while some IMAP servers are built on Unix or PC file
 systems, many others are not, and the file system semantics do not
 make sense in those configurations.  Second, while some IMAP servers
 expose the underlying file system to the clients, others allow access
 only to the user's personal mailboxes, or to some other limited set
 of files, making such file-system-like semantics less meaningful.
 Third, because the IMAP spec leaves the interpretation of the
 reference name as "implementation-dependent", in the past the various
 server implementations handled it in vastly differing ways.

Leiba Informational [Page 17] RFC 2683 IMAP4 Implementation Recommendations September 1999

 The following recommendations are the result of significant
 operational experience, and are intended to maximize
 interoperability.
 Server implementations must implement the reference argument in a way
 that matches the intended "change directory" operation as closely as
 possible.  As a minimum implementation, the reference argument may be
 prepended to the mailbox name (while suppressing double delimiters;
 see the next paragraph).  Even servers that do not provide a way to
 break out of the current hierarchy (see "breakout facility" below)
 must provide a reasonable implementation of the reference argument,
 as described here, so that they will interoperate with clients that
 use it.
 Server implementations that prepend the reference argument to the
 mailbox name should insert a hierarchy delimiter between them, and
 must not insert a second if one is already present:
     C: A001 LIST ABC DEF
     S: * LIST () "/" ABC/DEF   <=== should do this
     S: A001 OK done
     C: A002 LIST ABC/ /DEF
     S: * LIST () "/" ABC//DEF     <=== must not do this
     S: A002 OK done
 On clients, the reference argument is chiefly used to implement a
 "breakout facility", wherein the user may directly access a mailbox
 outside the "current directory" hierarchy.  Client implementations
 should have an operational mode that does not use the reference
 argument.  This is to interoperate with older servers that did not
 implement the reference argument properly.  While it's a good idea to
 give the user access to a breakout facility, clients that do not
 intend to do so should not use the reference argument at all.
 Client implementations should always place a trailing hierarchy
 delimiter on the reference argument.  This is because some servers
 prepend the reference argument to the mailbox name without inserting
 a hierarchy delimiter, while others do insert a hierarchy delimiter
 if one is not already present.  A client that puts the delimiter in
 will work with both varieties of server.
 Client implementations that implement a breakout facility should
 allow the user to choose whether or not to use a leading hierarchy
 delimiter on the mailbox argument.  This is because the handling of a
 leading mailbox hierarchy delimiter also varies from server to
 server, and even between different mailstores on the same server.  In
 some cases, a leading hierarchy delimiter means "discard the

Leiba Informational [Page 18] RFC 2683 IMAP4 Implementation Recommendations September 1999

 reference argument" (implementing the intended breakout facility),
 thus:
     C: A001 LIST ABC/ /DEF
     S: * LIST () "/" /DEF
     S: A001 OK done
 In other cases, however, the two are catenated and the extra
 hierarchy delimiter is discarded, thus:
     C: A001 LIST ABC/ /DEF
     S: * LIST () "/" ABC/DEF
     S: A001 OK done
 Client implementations must not assume that the server supports a
 breakout facility, but may provide a way for the user to use one if
 it is available.  Any breakout facility should be exported to the
 user interface.  Note that there may be other "breakout" characters
 besides the hierarchy delimiter (for instance, UNIX filesystem
 servers are likely to use a leading "~" as well), and that their
 interpretation is server-dependent.

3.4.10. Mailbox Hierarchy Delimiters

 The server's selection of what to use as a mailbox hierarchy
 delimiter is a difficult one, involving several issues: What
 characters do users expect to see?  What characters can they enter
 for a hierarchy delimiter if it is desired (or required) that the
 user enter it?  What character can be used for the hierarchy
 delimiter, noting that the chosen character can not otherwise be used
 in the mailbox name?
 Because some interfaces show users the hierarchy delimiters or allow
 users to enter qualified mailbox names containing them, server
 implementations should use delimiter characters that users generally
 expect to see as name separators.  The most common characters used
 for this are "/" (as in Unix file names), "\" (as in OS/2 and Windows
 file names), and "." (as in news groups).  There is little to choose
 among these apart from what users may expect or what is dictated by
 the underlying file system, if any.  One consideration about using
 "\" is that it's also a special character in the IMAP protocol. While
 the use of other hierarchy delimiter characters is permissible, A
 DESIGNER IS WELL ADVISED TO STAY WITH ONE FROM THIS SET unless the
 server is intended for special purposes only.  Implementers might be
 thinking about using characters such as "-", "_", ";", "&", "#", "@",
 and "!", but they should be aware of the surprise to the user as well
 as of the effect on URLs and other external specifications (since
 some of these characters have special meanings there).  Also, a

Leiba Informational [Page 19] RFC 2683 IMAP4 Implementation Recommendations September 1999

 server that uses "\" (and clients of such a server) must remember to
 escape that character in quoted strings or to send literals instead.
 Literals are recommended over escaped characters in quoted strings in
 order to maintain compatibility with older IMAP versions that did not
 allow escaped characters in quoted strings (but check the grammar to
 see where literals are allowed):
     C: 001 LIST "" {13}
     S: + send literal
     C: this\%\%\%\h*
     S: * LIST () "\\" {27}
     S: this\is\a\mailbox\hierarchy
     S: 001 OK LIST complete
 In any case, a server should not use normal alpha-numeric characters
 (such as "X" or "0") as delimiters; a user would be very surprised to
 find that "EXPENDITURES" actually represented a two-level hierarchy.
 And a server should not use characters that are non-printable or
 difficult or impossible to enter on a standard US keyboard.  Control
 characters, box-drawing characters, and characters from non-US
 alphabets fit into this category.  Their use presents
 interoperability problems that are best avoided.
 The UTF-7 encoding of mailbox names also raises questions about what
 to do with the hierarchy delimiters in encoded names: do we encode
 each hierarchy level and separate them with delimiters, or do we
 encode the fully qualified name, delimiters and all?  The answer for
 IMAP is the former: encode each hierarchy level separately, and
 insert delimiters between.  This makes it particularly important not
 to use as a hierarchy delimiter a character that might cause
 confusion with IMAP's modified UTF-7 [UTF-7 and RFC-2060] encoding.
 To repeat: a server should use "/", "\", or "." as its hierarchy
 delimiter.  The use of any other character is likely to cause
 problems and is STRONGLY DISCOURAGED.

3.4.11. ALERT Response Codes

 The protocol spec is very clear on the matter of what to do with
 ALERT response codes, and yet there are many clients that violate it
 so it needs to be said anyway: "The human-readable text contains a
 special alert that must be presented to the user in a fashion that
 calls the user's attention to the message."  That should be clear
 enough, but I'll repeat it here: Clients must present ALERT text
 clearly to the user.

Leiba Informational [Page 20] RFC 2683 IMAP4 Implementation Recommendations September 1999

3.4.12. Deleting Mailboxes

 The protocol does not guarantee that a client may delete a mailbox
 that is not empty, though on some servers it is permissible and is,
 in fact, much faster than the alternative or deleting all the
 messages from the client.  If the client chooses to try to take
 advantage of this possibility it must be prepared to use the other
 method in the even that the more convenient one fails.  Further, a
 client should not try to delete the mailbox that it has selected, but
 should first close that mailbox; some servers do not permit the
 deletion of the selected mailbox.
 That said, a server should permit the deletion of a non-empty
 mailbox; there's little reason to pass this work on to the client.
 Moreover, forbidding this prevents the deletion of a mailbox that for
 some reason can not be opened or expunged, leading to possible
 denial-of-service problems.
 Example:
     [User tells the client to delete mailbox BANANA, which is
     currently selected...]
     C: 008 CLOSE
     S: 008 OK done
     C: 009 DELETE BANANA
     S: 009 NO Delete failed; mailbox is not empty.
     C: 010 SELECT BANANA
     S: * ... untagged SELECT responses
     S: 010 OK done
     C: 011 STORE 1:* +FLAGS.SILENT \DELETED
     S: 011 OK done
     C: 012 CLOSE
     S: 012 OK done
     C: 013 DELETE BANANA
     S: 013 OK done

3.5. A Word About Testing

 Since the whole point of IMAP is interoperability, and since
 interoperability can not be tested in a vacuum, the final
 recommendation of this treatise is, "Test against EVERYTHING."  Test
 your client against every server you can get an account on.  Test
 your server with every client you can get your hands on.  Many
 clients make limited test versions available on the Web for the
 downloading.  Many server owners will give serious client developers
 guest accounts for testing.  Contact them and ask.  NEVER assume that
 because your client works with one or two servers, or because your
 server does fine with one or two clients, you will interoperate well

Leiba Informational [Page 21] RFC 2683 IMAP4 Implementation Recommendations September 1999

 in general.
 In particular, in addition to everything else, be sure to test
 against the reference implementations: the PINE client, the
 University of Washington server, and the Cyrus server.
 See the following URLs on the web for more information here:
     IMAP Products and Sources: http://www.imap.org/products.html
     IMC MailConnect: http://www.imc.org/imc-mailconnect

4. Security Considerations

 This document describes behaviour of clients and servers that use the
 IMAP4 protocol, and as such, has the same security considerations as
 described in [RFC-2060].

5. References

 [RFC-2060]  Crispin, M., "Internet Message Access Protocol - Version
             4rev1", RFC 2060, December 1996.
 [RFC-2119]  Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC-2180]  Gahrns, M., "IMAP4 Multi-Accessed Mailbox Practice", RFC
             2180, July 1997.
 [UTF-8]     Yergeau, F., " UTF-8, a transformation format of Unicode
             and ISO 10646", RFC 2044, October 1996.
 [UTF-7]     Goldsmith, D. and M. Davis, "UTF-7, a Mail-Safe
             Transformation Format of Unicode", RFC 2152, May 1997.
 [NAMESPACE] Gahrns, M. and C. Newman, "IMAP4 Namespace", Work in
             Progress.

6. Author's Address

 Barry Leiba
 IBM T.J. Watson Research Center
 30 Saw Mill River Road
 Hawthorne, NY  10532
 Phone: 1-914-784-7941
 EMail: leiba@watson.ibm.com

Leiba Informational [Page 22] RFC 2683 IMAP4 Implementation Recommendations September 1999

7. Full Copyright Statement

 Copyright (C) The Internet Society (1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Leiba Informational [Page 23]

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