rfc:rfc1460
Network Working Group M. Rose Request for Comments: 1460 Dover Beach Consulting, Inc. Obsoletes: 1225 June 1993
Post Office Protocol - Version 3
Status of this Memo
This RFC specifies an IAB standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "IAB Official Protocol Standards" for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Overview
This memo is a revision to RFC 1225, a Draft Standard. It makes the following changes from that document:
- the RPOP facility is removed;
- the optional APOP facility is added (which is in interoperable,
operational use in at least three implementations);
- a typo was corrected with respect to the interaction of LAST
and RSET;
- section numbers were added; and,
- an acknowledgements section was added.
1. Introduction
On certain types of smaller nodes in the Internet it is often impractical to maintain a message transport system (MTS). For example, a workstation may not have sufficient resources (cycles, disk space) in order to permit a SMTP server [RFC821] and associated local mail delivery system to be kept resident and continuously running.
Similarly, it may be expensive (or impossible) to keep a personal computer interconnected to an IP-style network for long amounts of time (the node is lacking the resource known as "connectivity").
Despite this, it is often very useful to be able to manage mail on these smaller nodes, and they often support a user agent (UA) to aid
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the tasks of mail handling. To solve this problem, a node which can support an MTS entity offers a maildrop service to these less endowed nodes. The Post Office Protocol - Version 3 (POP3) is intended to permit a workstation to dynamically access a maildrop on a server host in a useful fashion. Usually, this means that the POP3 is used to allow a workstation to retrieve mail that the server is holding for it.
For the remainder of this memo, the term "client host" refers to a host making use of the POP3 service, while the term "server host" refers to a host which offers the POP3 service.
2. A Short Digression
This memo does not specify how a client host enters mail into the transport system, although a method consistent with the philosophy of this memo is presented here:
When the user agent on a client host wishes to enter a message
into the transport system, it establishes an SMTP connection to
its relay host (this relay host could be, but need not be, the
POP3 server host for the client host).
If this method is followed, then the client host appears to the MTS as a user agent, and should NOT be regarded as a "trusted" MTS entity in any sense whatsoever. This concept, along with the role of the POP3 as a part of a split-UA model is discussed later in this memo.
3. Basic Operation
Initially, the server host starts the POP3 service by listening on TCP port 110. When a client host wishes to make use of the service, it establishes a TCP connection with the server host. When the connection is established, the POP3 server sends a greeting. The client and POP3 server then exchange commands and responses (respectively) until the connection is closed or aborted.
Commands in the POP3 consist of a keyword possibly followed by an argument. All commands are terminated by a CRLF pair.
Responses in the POP3 consist of a success indicator and a keyword
possibly followed by additional information. All responses are
terminated by a CRLF pair. There are currently two success
indicators: positive ("+OK") and negative ("-ERR").
Responses to certain commands are multi-line. In these cases, which are clearly indicated below, after sending the first line of the response and a CRLF, any additional lines are sent, each terminated
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by a CRLF pair. When all lines of the response have been sent, a final line is sent, consisting of a termination octet (decimal code 046, ".") and a CRLF pair. If any line of the multi-line response begins with the termination octet, the line is "byte-stuffed" by pre-pending the termination octet to that line of the response.
Hence a multi-line response is terminated with the five octets "CRLF.CRLF". When examining a multi-line response, the client checks to see if the line begins with the termination octet. If so and if octets other than CRLF follow, the the first octet of the line (the termination octet) is stripped away. If so and if CRLF immediately follows the termination character, then the response from the POP server is ended and the line containing ".CRLF" is not considered part of the multi-line response.
A POP3 session progresses through a number of states during its lifetime. Once the TCP connection has been opened and the POP3 server has sent the greeting, the session enters the AUTHORIZATION state. In this state, the client must identify itself to the POP3 server. Once the client has successfully done this, the server acquires resources associated with the client's maildrop, and the session enters the TRANSACTION state. In this state, the client requests actions on the part of the POP3 server. When the client has finished its transactions, the session enters the UPDATE state. In this state, the POP3 server releases any resources acquired during the TRANSACTION state and says goodbye. The TCP connection is then closed.
4. The AUTHORIZATION State
Once the TCP connection has been opened by a POP3 client, the POP3 server issues a one line greeting. This can be any string terminated by CRLF. An example might be:
S. +OK POP3 server ready
Note that this greeting is a POP3 reply. The POP3 server should always give a positive response as the greeting.
The POP3 session is now in the AUTHORIZATION state. The client must
now issue the USER command. If the POP3 server responds with a
positive success indicator ("+OK"), then the client may issue either
the PASS command to complete the authorization, or the QUIT command
to terminate the POP3 session. If the POP3 server responds with a
negative success indicator ("-ERR") to the USER command, then the
client may either issue a new USER command or may issue the QUIT
command.
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When the client issues the PASS command, the POP3 server uses the
argument pair from the USER and PASS commands to determine if the
client should be given access to the appropriate maildrop. If so,
the POP3 server then acquires an exclusive-access lock on the
maildrop. If the lock is successfully acquired, the POP3 server
parses the maildrop into individual messages (read note below),
determines the last message (if any) present in the maildrop that was
referenced by the RETR command, and responds with a positive success
indicator. The POP3 session now enters the TRANSACTION state. If
the lock can not be acquired or the client should is denied access to
the appropriate maildrop or the maildrop can't be parsed for some
reason, the POP3 server responds with a negative success indicator.
(If a lock was acquired but the POP3 server intends to respond with a
negative success indicator, the POP3 server must release the lock
prior to rejecting the command.) At this point, the client may
either issue a new USER command and start again, or the client may
issue the QUIT command.
NOTE: Minimal implementations of the POP3 need only be
able to break a maildrop into its component messages;
they need NOT be able to parse individual messages.
More advanced implementations may wish to have this
capability, for reasons discussed later.
After the POP3 server has parsed the maildrop into individual messages, it assigns a message-id to each message, and notes the size of the message in octets. The first message in the maildrop is assigned a message-id of "1", the second is assigned "2", and so on, so that the n'th message in a maildrop is assigned a message-id of "n". In POP3 commands and responses, all message-id's and message sizes are expressed in base-10 (i.e., decimal).
It sets the "highest number accessed" to be that of the last message referenced by the RETR command.
Here are summaries for the three POP3 commands discussed thus far:
USER name
Arguments: a server specific user-id (required)
Restrictions: may only be given in the AUTHORIZATION
state after the POP3 greeting or after an
unsuccessful USER or PASS command
Possible Responses:
+OK name is welcome here
-ERR never heard of name
Examples:
C: USER mrose
S: +OK mrose is a real hoopy frood
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...
C: USER frated
S: -ERR sorry, frated doesn't get his mail here
PASS string
Arguments: a server/user-id specific password (required)
Restrictions: may only be given in the AUTHORIZATION
state after a successful USER command
Possible Responses:
+OK maildrop locked and ready
-ERR invalid password
-ERR unable to lock maildrop
Examples:
C: USER mrose
S: +OK mrose is a real hoopy frood
C: PASS secret
S: +OK mrose's maildrop has 2 messages
(320 octets)
...
C: USER mrose
S: +OK mrose is a real hoopy frood
C: PASS secret
S: -ERR unable to lock mrose's maildrop, file
already locked
QUIT
Arguments: none
Restrictions: none
Possible Responses:
+OK
Examples:
C: QUIT
S: +OK dewey POP3 server signing off
5. The TRANSACTION State
Once the client has successfully identified itself to the POP3 server and the POP3 server has locked and burst the appropriate maildrop, the POP3 session is now in the TRANSACTION state. The client may now issue any of the following POP3 commands repeatedly. After each command, the POP3 server issues a response. Eventually, the client issues the QUIT command and the POP3 session enters the UPDATE state.
Here are the POP3 commands valid in the TRANSACTION state:
STAT
Arguments: none
Restrictions: may only be given in the TRANSACTION state.
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Discussion:
The POP3 server issues a positive response with a line
containing information for the maildrop. This line is
called a "drop listing" for that maildrop.
In order to simplify parsing, all POP3 servers are
required to use a certain format for drop listings.
The first octets present must indicate the number of
messages in the maildrop. Following this is the size
of the maildrop in octets. This memo makes no
requirement on what follows the maildrop size.
Minimal implementations should just end that line of
the response with a CRLF pair. More advanced
implementations may include other information.
NOTE: This memo STRONGLY discourages
implementations from supplying additional
information in the drop listing. Other,
optional, facilities are discussed later on
which permit the client to parse the messages
in the maildrop.
Note that messages marked as deleted are not counted in
either total.
Possible Responses:
+OK nn mm
Examples:
C: STAT
S: +OK 2 320
LIST [msg]
Arguments: a message-id (optionally) If a message-id is
given, it may NOT refer to a message marked as
deleted.
Restrictions: may only be given in the TRANSACTION state.
Discussion:
If an argument was given and the POP3 server issues a
positive response with a line containing information
for that message. This line is called a "scan listing"
for that message.
If no argument was given and the POP3 server issues a positive response, then the response given is multi-line. After the initial +OK, for each message in the maildrop, the POP3 server responds with a line
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containing information for that message. This line
is called a "scan listing" for that message.
In order to simplify parsing, all POP3 servers are
required to use a certain format for scan listings.
The first octets present must be the message-id of
the message. Following the message-id is the size of
the message in octets. This memo makes no requirement
on what follows the message size in the scan listing.
Minimal implementations should just end that line of
the response with a CRLF pair. More advanced
implementations may include other information, as
parsed from the message.
NOTE: This memo STRONGLY discourages
implementations from supplying additional
information in the scan listing. Other, optional,
facilities are discussed later on which permit
the client to parse the messages in the maildrop.
Note that messages marked as deleted are not listed.
Possible Responses:
+OK scan listing follows
-ERR no such message
Examples:
C: LIST
S: +OK 2 messages (320 octets)
S: 1 120
S: 2 200
S: .
...
C: LIST 2
S: +OK 2 200
...
C: LIST 3
S: -ERR no such message, only 2 messages in
maildrop
RETR msg
Arguments: a message-id (required) This message-id may
NOT refer to a message marked as deleted.
Restrictions: may only be given in the TRANSACTION state.
Discussion:
If the POP3 server issues a positive response, then the
response given is multi-line. After the initial +OK,
the POP3 server sends the message corresponding to the
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given message-id, being careful to byte-stuff the
termination character (as with all multi-line
responses).
If the number associated with this message is higher
than the "highest number accessed" in the maildrop, the
POP3 server updates the "highest number accessed" to
the number associated with this message.
Possible Responses:
+OK message follows
-ERR no such message
Examples:
C: RETR 1
S: +OK 120 octets
S: <the POP3 server sends the entire message here>
S: .
DELE msg
Arguments: a message-id (required) This message-id
may NOT refer to a message marked as deleted.
Restrictions: may only be given in the TRANSACTION state.
Discussion:
The POP3 server marks the message as deleted. Any
future reference to the message-id associated with the
message in a POP3 command generates an error. The POP3
server does not actually delete the message until the
POP3 session enters the UPDATE state.
If the number associated with this message is higher
than the "highest number accessed" in the maildrop,
the POP3 server updates the "highest number accessed"
to the number associated with this message.
Possible Responses:
+OK message deleted
-ERR no such message
Examples:
C: DELE 1
S: +OK message 1 deleted
...
C: DELE 2
S: -ERR message 2 already deleted
NOOP
Arguments: none
Restrictions: may only be given in the TRANSACTION state.
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Discussion:
The POP3 server does nothing, it merely replies with a
positive response.
Possible Responses:
+OK
Examples:
C: NOOP
S: +OK
LAST
Arguments: none
Restrictions: may only be issued in the TRANSACTION state.
Discussion:
The POP3 server issues a positive response with a line
containing the highest message number which accessed.
Zero is returned in case no message in the maildrop has
been accessed during previous transactions. A client
may thereafter infer that messages, if any, numbered
greater than the response to the LAST command are
messages not yet accessed by the client.
Possible Response:
+OK nn
Examples:
C: STAT
S: +OK 4 320
C: LAST
S: +OK 1
C: RETR 3
S: +OK 120 octets
S: <the POP3 server sends the entire message
here>
S: .
C: LAST
S: +OK 3
C: DELE 2
S: +OK message 2 deleted
C: LAST
S: +OK 3
C: RSET
S: +OK
C: LAST
S: +OK 0
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RSET
Arguments: none
Restrictions: may only be given in the TRANSACTION
state.
Discussion:
If any messages have been marked as deleted by the POP3
server, they are unmarked. The POP3 server then
replies with a positive response. In addition, the
"highest number accessed" is also reset to zero.
Possible Responses:
+OK
Examples:
C: RSET
S: +OK maildrop has 2 messages (320 octets)
6. The UPDATE State
When the client issues the QUIT command from the TRANSACTION state, the POP3 session enters the UPDATE state. (Note that if the client issues the QUIT command from the AUTHORIZATION state, the POP3 session terminates but does NOT enter the UPDATE state.)
QUIT
Arguments: none
Restrictions: none
Discussion:
The POP3 server removes all messages marked as deleted
from the maildrop. It then releases the
exclusive-access lock on the maildrop and replies as
to the success of these operations. The TCP
connection is then closed.
Possible Responses:
+OK
Examples:
C: QUIT
S: +OK dewey POP3 server signing off (maildrop
empty)
...
C: QUIT
S: +OK dewey POP3 server signing off (2 messages
left)
...
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7. Optional POP3 Commands
The POP3 commands discussed above must be supported by all minimal implementations of POP3 servers.
The optional POP3 commands described below permit a POP3 client greater freedom in message handling, while preserving a simple POP3 server implementation.
NOTE: This memo STRONGLY encourages implementations to
support these commands in lieu of developing augmented
drop and scan listings. In short, the philosophy of
this memo is to put intelligence in the part of the
POP3 client and not the POP3 server.
TOP msg n
Arguments: a message-id (required) and a number. This
message-id may NOT refer to a message marked as
deleted.
Restrictions: may only be given in the TRANSACTION state.
Discussion:
If the POP3 server issues a positive response, then
the response given is multi-line. After the initial
+OK, the POP3 server sends the headers of the message,
the blank line separating the headers from the body,
and then the number of lines indicated message's body,
being careful to byte-stuff the termination character
(as with all multi-line responses).
Note that if the number of lines requested by the POP3
client is greater than than the number of lines in the
body, then the POP3 server sends the entire message.
Possible Responses:
+OK top of message follows
-ERR no such message
Examples:
C: TOP 10
S: +OK
S: <the POP3 server sends the headers of the
message, a blank line, and the first 10 lines
of the body of the message>
S: .
...
C: TOP 100
S: -ERR no such message
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APOP name digest
Arguments: a server specific user-id and a digest string
(both required).
Restrictions: may only be given in the AUTHORIZATION
state after the POP3 greeting
Discussion:
Normally, each POP3 session starts with a USER/PASS
exchange. This results in a server/user-id specific
password being sent in the clear on the network. For
intermittent use of POP3, this may not introduce a
sizable risk. However, many POP3 client
implementations connect to the POP3 server on a
regular basis -- to check for new mail. Further the
interval of session initiation may be on the order of
five minutes. Hence, the risk of password capture is
greatly enhanced.
An alternate method of authentication is required
which provides for both origin authentication and
replay protection, but which does not involve sending
a password in the clear over the network. The APOP
command provides this functionality.
A POP3 server which implements the APOP command will
include a timestamp in its banner greeting. The
syntax of the timestamp corresponds to the "msg-id"
in [RFC822], and MUST be different each time the POP3
server issues a banner greeting. For example, on a
UNIX implementation in which a separate UNIX process
is used for each instance of a POP3 server, the
syntax of the timestamp might be:
<process-ID.clock@hostname>
where "process-ID" is the decimal value of the
process's PID, clock is the decimal value of the
system clock, and hostname is the fully-qualified
domain-name corresponding to the host where the POP3
server is running.
The POP3 client makes note of this timestamp, and
then issues the APOP command. The "name" parameter
has identical semantics to the "name" parameter of
the USER command. The "digest" parameter is
calculated by applying the MD5 algorithm [RFC1321] to
a string consisting of the timestamp (including
angle-brackets) followed by a shared secret. This
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shared secret is a string known only to the POP3
client and server. Great care should be taken to
prevent unauthorized disclosure of the secret, as
knowledge of the secret will allow any entity to
successfully masquerade as the named user. The
"digest" parameter itself is a 16-octet value which
is sent in hexadecimal format, using lower-case ASCII
characters.
When the POP3 server receives the APOP command, it
verifies the digest provided. If the digest is
correct, the POP3 server issues a positive response,
and the POP3 session enters the TRANSACTION state.
Otherwise, a negative response is issued and the POP3
session remains in the AUTHORIZATION state.
Possible Responses:
+OK maildrop locked and ready
-ERR permission denied
Examples:
S: +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us>
C: APOP mrose c4c9334bac560ecc979e58001b3e22fb
S: +OK maildrop has 1 message (369 octets)
In this example, the shared secret is the string "tanstaaf".
Hence, the MD5 algorithm is applied to the string
<1896.697170952@dbc.mtview.ca.us>tanstaaf
which produces a digest value of
c4c9334bac560ecc979e58001b3e22fb
8. POP3 Command Summary
Minimal POP3 Commands:
USER name valid in the AUTHORIZATION state
PASS string
QUIT
STAT valid in the TRANSACTION state
LIST [msg]
RETR msg
DELE msg
NOOP
LAST
RSET
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QUIT valid in the UPDATE state
Optional POP3 Commands:
APOP name digest valid in the AUTHORIZATION state
TOP msg n valid in the TRANSACTION state
POP3 Replies:
+OK
-ERR
Note that with the exception of the STAT command, the reply given
by the POP3 server to any command is significant only to "+OK"
and "-ERR". Any text occurring after this reply may be ignored
by the client.
9. Example POP3 Session
S: <wait for connection on TCP port 110>
...
C: <open connection>
S: +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us>
C: APOP mrose c4c9334bac560ecc979e58001b3e22fb
S: +OK mrose's maildrop has 2 messages (320 octets)
C: STAT
S: +OK 2 320
C: LIST
S: +OK 2 messages (320 octets)
S: 1 120
S: 2 200
S: .
C: RETR 1
S: +OK 120 octets
S: <the POP3 server sends message 1>
S: .
C: DELE 1
S: +OK message 1 deleted
C: RETR 2
S: +OK 200 octets
S: <the POP3 server sends message 2>
S: .
C: DELE 2
S: +OK message 2 deleted
C: QUIT
S: +OK dewey POP3 server signing off (maildrop empty)
C: <close connection>
S: <wait for next connection>
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10. Message Format
All messages transmitted during a POP3 session are assumed to conform to the standard for the format of Internet text messages [RFC822].
It is important to note that the byte count for a message on the server host may differ from the octet count assigned to that message due to local conventions for designating end-of-line. Usually, during the AUTHORIZATION state of the POP3 session, the POP3 client can calculate the size of each message in octets when it parses the maildrop into messages. For example, if the POP3 server host internally represents end-of-line as a single character, then the POP3 server simply counts each occurrence of this character in a message as two octets. Note that lines in the message which start with the termination octet need not be counted twice, since the POP3 client will remove all byte-stuffed termination characters when it receives a multi-line response.
11. The POP and the Split-UA model
The underlying paradigm in which the POP3 functions is that of a split-UA model. The POP3 client host, being a remote PC based workstation, acts solely as a client to the message transport system. It does not provide delivery/authentication services to others. Hence, it is acting as a UA, on behalf of the person using the workstation. Furthermore, the workstation uses SMTP to enter mail into the MTS.
In this sense, we have two UA functions which interface to the message transport system: Posting (SMTP) and Retrieval (POP3). The entity which supports this type of environment is called a split-UA (since the user agent is split between two hosts which must interoperate to provide these functions).
ASIDE: Others might term this a remote-UA instead.
There are arguments supporting the use of both terms.
This memo has explicitly referenced TCP as the underlying transport agent for the POP3. This need not be the case. In the MZnet split- UA, for example, personal micro-computer systems are used which do not have IP-style networking capability [MZnet]. To connect to the POP3 server host, a PC establishes a terminal connection using some simple protocol (PhoneNet). A program on the PC drives the connection, first establishing a login session as a normal user. The login shell for this pseudo-user is a program which drives the other half of the terminal protocol and communicates with one of two servers. Although MZnet can support several PCs, a single pseudo- user login is present on the server host. The user-id and password
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for this pseudo-user login is known to all members of MZnet. Hence, the first action of the login shell, after starting the terminal protocol, is to demand a USER/PASS authorization pair from the PC. This second level of authorization is used to ascertain who is interacting with the MTS. Although the server host is deemed to support a "trusted" MTS entity, PCs in MZnet are not. Naturally, the USER/PASS authorization pair for a PC is known only to the owner of the PC (in theory, at least).
After successfully verifying the identity of the client, a modified SMTP server is started, and the PC posts mail with the server host. After the QUIT command is given to the SMTP server and it terminates, a modified POP3 server is started, and the PC retrieves mail from the server host. After the QUIT command is given to the POP3 server and it terminates, the login shell for the pseudo-user terminates the terminal protocol and logs the job out. The PC then closes the terminal connection to the server host.
The SMTP server used by MZnet is modified in the sense that it knows that it's talking to a user agent and not a "trusted" entity in the message transport system. Hence, it does performs the validation activities normally performed by an entity in the MTS when it accepts a message from a UA.
The POP3 server used by MZnet is modified in the sense that it does not require a USER/PASS combination before entering the TRANSACTION state. The reason for this (of course) is that the PC has already identified itself during the second-level authorization step described above.
NOTE: Truth in advertising laws require that the author
of this memo state that MZnet has not actually been
fully implemented. The concepts presented and proven
by the project led to the notion of the MZnet
split-slot model. This notion has inspired the
split-UA concept described in this memo, led to the
author's interest in the POP, and heavily influenced
the the description of the POP3 herein.
In fact, some UAs present in the Internet already support the notion of posting directly to an SMTP server and retrieving mail directly from a POP3 server, even if the POP3 server and client resided on the same host!
ASIDE: this discussion raises an issue which this memo
purposedly avoids: how does SMTP know that it's talking
to a "trusted" MTS entity?
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12. References
[MZnet] Stefferud, E., Sweet, J., and T. Domae, "MZnet: Mail
Service for Personal Micro-Computer Systems,:
Proceedings, IFIP 6.5 International Conference on
Computer Message Systems, Nottingham, U.K., May 1984.
[RFC821] Postel, J., "Simple Mail Transfer Protocol", STD 10,
RFC 821, USC/Information Sciences Institute, August 1982.
[RFC822] Crocker, D., "Standard for the Format of ARPA-Internet
Text Messages", STD 11, RFC 822, University of Delaware,
August 1982.
[RFC1321] Rivest, R. "The MD5 Message-Digest Algorithm", MIT
Laboratory for Computer Science, April 1992.
13. Security Considerations
It is conjectured that use of the APOP command provides origin identification and replay protection for a POP3 session. Accordingly, a POP3 server which implements both the PASS and APOP commands must not allow both methods of access for a given user; that is, for a given "USER name" either the PASS or APOP command is allowed, but not both.
Otherwise, security issues are not discussed in this memo.
14. Acknowledgements
The POP family has a long and checkered history. Although primarily a minor revision to [RFC1225], POP3 is based on the ideas presented in RFCs 918, 937, and 1081.
In addition, Alfred Grimstad, Keith McCloghrie, and Neil Ostroff provided significant comments on the APOP command.
15. Author's Address
Marshall T. Rose Dover Beach Consulting, Inc. Mountain View, CA 94043-2186
Phone: +1 415 968 1052 Fax: +1 415 968 2510
EMail: mrose@dbc.mtview.ca.us X.500: rose, dbc, us
Rose [Page 17]
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