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rfc:rfc1869

Network Working Group J. Klensin, WG Chair Request For Comments: 1869 MCI STD: 10 N. Freed, Editor Obsoletes: 1651 Innosoft International, Inc. Category: Standards Track M. Rose

                                          Dover Beach Consulting, Inc.
                                                          E. Stefferud
                                   Network Management Associates, Inc.
                                                            D. Crocker
                                                Brandenburg Consulting
                                                         November 1995
                      SMTP Service Extensions

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

1. Abstract

 This memo defines a framework for extending the SMTP service by
 defining a means whereby a server SMTP can inform a client SMTP as to
 the service extensions it supports.  Extensions to the SMTP service
 are registered with the IANA. This framework does not require
 modification of existing SMTP clients or servers unless the features
 of the service extensions are to be requested or provided.

2. Introduction

 The Simple Mail Transfer Protocol (SMTP) [1] has provided a stable,
 effective basis for the relay function of message transfer agents.
 Although a decade old, SMTP has proven remarkably resilient.
 Nevertheless, the need for a number of protocol extensions has become
 evident. Rather than describing these extensions as separate and
 haphazard entities, this document enhances SMTP in a straightforward
 fashion that provides a framework in which all future extensions can
 be built in a single consistent way.

3. Framework for SMTP Extensions

 For the purpose of service extensions to SMTP, SMTP relays a mail
 object containing an envelope and a content.

Klensin, et al Standards Track [Page 1] RFC 1869 SMTP Service Extensions November 1995

(1) The SMTP envelope is straightforward, and is sent as a

     series of SMTP protocol units: it consists of an
     originator address (to which error reports should be
     directed); a delivery mode (e.g., deliver to recipient
     mailboxes); and, one or more recipient addresses.

(2) The SMTP content is sent in the SMTP DATA protocol unit

     and has two parts: the headers and the body. The
     headers form a collection of field/value pairs
     structured according to RFC 822 [2], whilst the body,
     if structured, is defined according to MIME [3]. The
     content is textual in nature, expressed using the US
     ASCII repertoire (ANSI X3.4-1986). Although extensions
     (such as MIME) may relax this restriction for the
     content body, the content headers are always encoded
     using the US ASCII repertoire. The algorithm defined in
     [4] is used to represent header values outside the US
     ASCII repertoire, whilst still encoding them using the
     US ASCII repertoire.
 Although SMTP is widely and robustly deployed, some parts of the
 Internet community might wish to extend the SMTP service.  This memo
 defines a means whereby both an extended SMTP client and server may
 recognize each other as such and the server can inform the client as
 to the service extensions that it supports.
 It must be emphasized that any extension to the SMTP service should
 not be considered lightly. SMTP's strength comes primarily from its
 simplicity.  Experience with many protocols has shown that:
   protocols with few options tend towards ubiquity, whilst
   protocols with many options tend towards obscurity.
 This means that each and every extension, regardless of its benefits,
 must be carefully scrutinized with respect to its implementation,
 deployment, and interoperability costs. In many cases, the cost of
 extending the SMTP service will likely outweigh the benefit.
 Given this environment, the framework for the extensions described in
 this memo consists of:

(1) a new SMTP command (section 4)

(2) a registry of SMTP service extensions (section 5)

(3) additional parameters to the SMTP MAIL FROM and RCPT TO

     commands (section 6).

Klensin, et al Standards Track [Page 2] RFC 1869 SMTP Service Extensions November 1995

4. The EHLO command

 A client SMTP supporting SMTP service extensions should start an SMTP
 session by issuing the EHLO command instead of the HELO command. If
 the SMTP server supports the SMTP service extensions it will give a
 successful response (see section 4.3), a failure response (see 4.4),
 or an error response (4.5). If the SMTP server does not support any
 SMTP service extensions it will generate an error response (see
 section 4.5).

4.1. Changes to STD 10, RFC 821

 This specification is intended to extend STD 10, RFC 821 without
 impacting existing services in any way.  The minor changes needed are
 enumerated below.

4.1.1. First command

 RFC 821 states that the first command in an SMTP session must be the
 HELO command. This requirement is hereby amended to allow a session
 to start with either EHLO or HELO.

4.1.2. Maximum command line length

 This specification extends the SMTP MAIL FROM and RCPT TO to allow
 additional parameters and parameter values.  It is possible that the
 MAIL FROM and RCPT TO lines that result will exceed the 512 character
 limit on command line length imposed by RFC 821.  This limit is
 hereby amended to only apply to command lines without any parameters.
 Each specification that defines new MAIL FROM or RCPT TO parameters
 must also specify maximum parameter value lengths for each parameter
 so that implementors of some set of extensions know how much buffer
 space must be allocated. The maximum command length that must be
 supported by an SMTP implementation with extensions is 512 plus the
 sum of all the maximum parameter lengths for all the extensions
 supported.

4.2. Command syntax

 The syntax for this command, using the ABNF notation of [2], is:
   ehlo-cmd ::= "EHLO" SP domain CR LF
 If successful, the server SMTP responds with code 250. On failure,
 the server SMTP responds with code 550. On error, the server SMTP
 responds with one of codes 500, 501, 502, 504, or 421.

Klensin, et al Standards Track [Page 3] RFC 1869 SMTP Service Extensions November 1995

 This command is issued instead of the HELO command, and may be issued
 at any time that a HELO command would be appropriate.  That is, if
 the EHLO command is issued, and a successful response is returned,
 then a subsequent HELO or EHLO command will result in the server SMTP
 replying with code 503.  A client SMTP must not cache any information
 returned if the EHLO command succeeds. That is, a client SMTP must
 issue the EHLO command at the start of each SMTP session if
 information about extended facilities is needed.

4.3. Successful response

 If the server SMTP implements and is able to perform the EHLO
 command, it will return code 250.  This indicates that both the
 server and client SMTP are in the initial state, that is, there is no
 transaction in progress and all state tables and buffers are cleared.
 Normally, this response will be a multiline reply. Each line of the
 response contains a keyword and, optionally, one or more parameters.
 The syntax for a positive response, using the ABNF notation of [2],
 is:
   ehlo-ok-rsp  ::=      "250"    domain [ SP greeting ] CR LF
                  / (    "250-"   domain [ SP greeting ] CR LF
                      *( "250-"      ehlo-line           CR LF )
                         "250"    SP ehlo-line           CR LF   )
                ; the usual HELO chit-chat
   greeting     ::= 1*<any character other than CR or LF>
   ehlo-line    ::= ehlo-keyword *( SP ehlo-param )
   ehlo-keyword ::= (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
                ; syntax and values depend on ehlo-keyword
   ehlo-param   ::= 1*<any CHAR excluding SP and all
                       control characters (US ASCII 0-31
                       inclusive)>
   ALPHA        ::= <any one of the 52 alphabetic characters
                     (A through Z in upper case, and,
                      a through z in lower case)>
   DIGIT        ::= <any one of the 10 numeric characters
                     (0 through 9)>
   CR           ::= <the carriage-return character
                     (ASCII decimal code 13)>
   LF           ::= <the line-feed character
                     (ASCII decimal code 10)>

Klensin, et al Standards Track [Page 4] RFC 1869 SMTP Service Extensions November 1995

   SP           ::= <the space character
                     (ASCII decimal code 32)>
 Although EHLO keywords may be specified in upper, lower, or mixed
 case, they must always be recognized and processed in a case-
 insensitive manner. This is simply an extension of practices begun in
 RFC 821.
 The IANA maintains a registry of SMTP service extensions.  Associated
 with each such extension is a corresponding EHLO keyword value. Each
 service extension registered with the IANA must be defined in an RFC.
 Such RFCs must either be on the standards-track or must define an
 IESG-approved experimental protocol.  The definition must include:

(1) the textual name of the SMTP service extension;

(2) the EHLO keyword value associated with the extension;

(3) the syntax and possible values of parameters associated

     with the EHLO keyword value;

(4) any additional SMTP verbs associated with the extension

     (additional verbs will usually be, but are not required
     to be, the same as the EHLO keyword value);

(5) any new parameters the extension associates with the

     MAIL FROM or RCPT TO verbs;

(6) how support for the extension affects the behavior of a

     server and client SMTP; and,

(7) the increment by which the extension is increasing the

     maximum length of the commands MAIL FROM, RCPT TO, or
     both, over that specified in RFC 821.
 In addition, any EHLO keyword value that starts with an upper or
 lower case "X" refers to a local SMTP service extension, which is
 used through bilateral, rather than standardized, agreement. Keywords
 beginning with "X" may not be used in a registered service extension.
 Any keyword values presented in the EHLO response that do not begin
 with "X" must correspond to a standard, standards-track, or IESG-
 approved experimental SMTP service extension registered with IANA.  A
 conforming server must not offer non "X" prefixed keyword values that
 are not described in a registered extension.

Klensin, et al Standards Track [Page 5] RFC 1869 SMTP Service Extensions November 1995

 Additional verbs are bound by the same rules as EHLO keywords;
 specifically, verbs begining with "X" are local extensions that may
 not be registered or standardized and verbs not beginning with "X"
 must always be registered.

4.4. Failure response

 If for some reason the server SMTP is unable to list the service
 extensions it supports, it will return code 554.
 In the case of a failure response, the client SMTP should issue
 either the HELO or QUIT command.

4.5. Error responses from extended servers

 If the server SMTP recognizes the EHLO command, but the command
 argument is unacceptable, it will return code 501.
 If the server SMTP recognizes, but does not implement, the EHLO
 command, it will return code 502.
 If the server SMTP determines that the SMTP service is no longer
 available (e.g., due to imminent system shutdown), it will return
 code 421.
 In the case of any error response, the client SMTP should issue
 either the HELO or QUIT command.

4.6. Responses from servers without extensions

 A server SMTP that conforms to RFC 821 but does not support the
 extensions specified here will not recognize the EHLO command and
 will consequently return code 500, as specified in RFC 821.  The
 server SMTP should stay in the same state after returning this code
 (see section 4.1.1 of RFC 821).  The client SMTP may then issue
 either a HELO or a QUIT command.

4.7. Responses from improperly implemented servers

 Some SMTP servers are known to disconnect the SMTP transmission
 channel upon receipt of the EHLO command. The disconnect can occur
 immediately or after sending a response.  Such behavior violates
 section 4.1.1 of RFC 821, which explicitly states that disconnection
 should only occur after a QUIT command is issued.
 Nevertheless, in order to achieve maxmimum interoperablity it is
 suggested that extended SMTP clients using EHLO be coded to check for
 server connection closure after EHLO is sent, either before or after

Klensin, et al Standards Track [Page 6] RFC 1869 SMTP Service Extensions November 1995

 returning a reply.  If this happens the client must decide if the
 operation can be successfully completed without using any SMTP
 extensions. If it can a new connection can be opened and the HELO
 command can be used.
 Other improperly-implemented servers will not accept a HELO command
 after EHLO has been sent and rejected.  In some cases, this problem
 can be worked around by sending a RSET after the failure response to
 EHLO, then sending the HELO.  Clients that do this should be aware
 that many implementations will return a failure code (e.g., 503 Bad
 sequence of commands) in response to the RSET.  This code can be
 safely ignored.

5. Initial IANA Registry

 The IANA's initial registry of SMTP service extensions consists of
 these entries:
 Service Ext   EHLO Keyword Parameters Verb       Added Behavior
 ------------- ------------ ---------- ---------- ------------------
 Send             SEND         none       SEND    defined in RFC 821
 Send or Mail     SOML         none       SOML    defined in RFC 821
 Send and Mail    SAML         none       SAML    defined in RFC 821
 Expand           EXPN         none       EXPN    defined in RFC 821
 Help             HELP         none       HELP    defined in RFC 821
 Turn             TURN         none       TURN    defined in RFC 821
 which correspond to those SMTP commands which are defined as optional
 in [5].  (The mandatory SMTP commands, according to [5], are HELO,
 MAIL, RCPT, DATA, RSET, VRFY, NOOP, and QUIT.)

6. MAIL FROM and RCPT TO Parameters

 It is recognized that several of the extensions planned for SMTP will
 make use of additional parameters associated with the MAIL FROM and
 RCPT TO command. The syntax for these commands, again using the ABNF
 notation of [2] as well as underlying definitions from [1], is:
   esmtp-cmd        ::= inner-esmtp-cmd [SP esmtp-parameters] CR LF
   esmtp-parameters ::= esmtp-parameter *(SP esmtp-parameter)
   esmtp-parameter  ::= esmtp-keyword ["=" esmtp-value]
   esmtp-keyword    ::= (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
                        ; syntax and values depend on esmtp-keyword
   esmtp-value      ::= 1*<any CHAR excluding "=", SP, and all
                           control characters (US ASCII 0-31
                           inclusive)>

Klensin, et al Standards Track [Page 7] RFC 1869 SMTP Service Extensions November 1995

                        ; The following commands are extended to
                        ; accept extended parameters.
   inner-esmtp-cmd  ::= ("MAIL FROM:" reverse-path)   /
                        ("RCPT TO:" forward-path)
 All esmtp-keyword values must be registered as part of the IANA
 registration process described above. This definition only provides
 the framework for future extension; no extended MAIL FROM or RCPT TO
 parameters are defined by this RFC.

6.1. Error responses

 If the server SMTP does not recognize or cannot implement one or more
 of the parameters associated with a particular MAIL FROM or RCPT TO
 command, it will return code 555.
 If for some reason the server is temporarily unable to accomodate one
 or more of the parameters associated with a MAIL FROM or RCPT TO
 command, and if the definition of the specific parameter does not
 mandate the use of another code, it should return code 455.
 Errors specific to particular parameters and their values will be
 specified in the parameter's defining RFC.

7. Received: Header Field Annotation

 SMTP servers are required to add an appropriate Received: field to
 the headers of all messages they receive. A "with ESMTP" clause
 should be added to this field when any SMTP service extensions are
 used. "ESMTP" is hereby added to the list of standard protocol names
 registered with IANA.

8. Usage Examples

(1) An interaction of the form:

     S: <wait for connection on TCP port 25>
     C: <open connection to server>
     S: 220 dbc.mtview.ca.us SMTP service ready
     C: EHLO ymir.claremont.edu
     S: 250 dbc.mtview.ca.us says hello
      ...
     indicates that the server SMTP implements only those
     SMTP commands which are defined as mandatory in [5].

Klensin, et al Standards Track [Page 8] RFC 1869 SMTP Service Extensions November 1995

(2) In contrast, an interaction of the form:

     S: <wait for connection on TCP port 25>
     C: <open connection to server>
     S: 220 dbc.mtview.ca.us SMTP service ready
     C: EHLO ymir.claremont.edu
     S: 250-dbc.mtview.ca.us says hello
     S: 250-EXPN
     S: 250-HELP
     S: 250-8BITMIME
     S: 250-XONE
     S: 250 XVRB
      ...
     indicates that the server SMTP also implements the SMTP
     EXPN and HELP commands, one standard service extension
     (8BITMIME), and two nonstandard and unregistered
     service extensions (XONE and XVRB).

(3) Finally, a server that does not support SMTP service

     extensions would act as follows:
     S: <wait for connection on TCP port 25>
     C: <open connection to server>
     S: 220 dbc.mtview.ca.us SMTP service ready
     C: EHLO ymir.claremont.edu
     S: 500 Command not recognized: EHLO
      ...
     The 500 response indicates that the server SMTP does
     not implement the extensions specified here.  The
     client would normally send a HELO command and proceed
     as specified in RFC 821.   See section 4.7 for
     additional discussion.

9. Security Considerations

 This RFC does not discuss security issues and is not believed to
 raise any security issues not already endemic in electronic mail and
 present in fully conforming implementations of RFC-821.  It does
 provide an announcement of server mail capabilities via the response
 to the EHLO verb. However, all information provided by announcement
 of any of the initial set of service extensions defined by this RFC
 can be readily deduced by selective probing of the verbs required to
 transport and deliver mail. The security implications of service
 extensions described in other RFCs should be dealt with in those
 RFCs.

Klensin, et al Standards Track [Page 9] RFC 1869 SMTP Service Extensions November 1995

10. Acknowledgements

 This document represents a synthesis of the ideas of many people and
 reactions to the ideas and proposals of others.  Randall Atkinson,
 Craig Everhart, Risto Kankkunen, and Greg Vaudreuil contributed ideas
 and text sufficient to be considered co-authors.  Other important
 suggestions, text, or encouragement came from Harald Alvestrand, Jim
 Conklin, Mark Crispin, Frank da Cruz, 'Olafur Gudmundsson, Per
 Hedeland, Christian Huitma, Neil Katin, Eliot Lear, Harold A.
 Miller, Keith Moore, John Myers, Dan Oscarsson, Julian Onions, Rayan
 Zachariassen, and the contributions of the entire IETF SMTP Working
 Group. Of course, none of the individuals are necessarily responsible
 for the combination of ideas represented here. Indeed, in some cases,
 the response to a particular criticism was to accept the problem
 identification but to include an entirely different solution from the
 one originally proposed.

11. References

 [1] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
     USC/Information Sciences Institute, August 1982.
 [2] Crocker, D., "Standard for the Format of ARPA Internet Text
     Messages", STD 11, RFC 822, UDEL, August 1982.
 [3] Borenstein, N., and N. Freed, "Multipurpose Internet Mail
     Extensions", RFC 1521, Bellcore, Innosoft, September 1993.
 [4] Moore, K., "Representation of Non-ASCII Text in Internet Message
     Headers", RFC 1522, University of Tennessee, September 1993.
 [5] Braden, R., "Requirements for Internet Hosts - Application and
     Support", STD 3, RFC 1123, USC/Information Sciences Institute,
     October 1989.

12. Chair, Editor, and Author Addresses

 John Klensin, WG Chair
 MCI
 2100 Reston Parkway
 Reston, VA 22091
 Phone: +1 703 715-7361
 Fax: +1 703 715-7436
 EMail: klensin@mci.net

Klensin, et al Standards Track [Page 10] RFC 1869 SMTP Service Extensions November 1995

 Ned Freed, Editor
 Innosoft International, Inc.
 1050 East Garvey Avenue South
 West Covina, CA 91790
 USA
 Phone: +1 818 919 3600
 Fax: +1 818 919 3614
 EMail: ned@innosoft.com
 Marshall T. Rose
 Dover Beach Consulting, Inc.
 420 Whisman Court
 Moutain View, CA  94043-2186
 USA
 Phone: +1 415 968 1052
 Fax: +1 415 968 2510
 EMail: mrose@dbc.mtview.ca.us
 Einar A. Stefferud
 Network Management Associates, Inc.
 17301 Drey Lane
 Huntington Beach, CA, 92647-5615
 USA
 Phone: +1 714 842 3711
 Fax: +1 714 848 2091
 EMail: stef@nma.com
 Dave Crocker
 Brandenburg Consulting
 675 Spruce Dr.
 Sunnyvale, CA 94086 USA
 USA
 Phone: +1 408 246 8253
 Fax: +1 408 249 6205
 EMail: dcrocker@mordor.stanford.edu

Klensin, et al Standards Track [Page 11]

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