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rfc:rfc1425

Network Working Group J. Klensin, WG Chair Request for Comments: 1425 United Nations University

                                                      N. Freed, Editor
                                          Innosoft International, Inc.
                                                               M. Rose
                                          Dover Beach Consulting, Inc.
                                                          E. Stefferud
                                   Network Management Associates, Inc.
                                                            D. Crocker
                                                     The Branch Office
                                                         February 1993
                      SMTP Service Extensions

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.

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. Standard extensions to the SMTP
 service are registered with the Internet Assigned Numbers Authority
 (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, Freed, Rose, Stefferud & Crocker [Page 1] RFC 1425 SMTP Service Extensions February 1993

        (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, Freed, Rose, Stefferud & Crocker [Page 2] RFC 1425 SMTP Service Extensions February 1993

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 4.1), a failure response (see 4.2),
 or an error response (4.3). If the SMTP server does not support any
 SMTP service extensions it will generate an error response (see
 section 4.4).
 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.
 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.

4.1. 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:

Klensin, Freed, Rose, Stefferud & Crocker [Page 3] RFC 1425 SMTP Service Extensions February 1993

       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)>
       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 standard SMTP service extensions.
 Associated with each such extension is a corresponding EHLO keyword
 value. Each service extension registered with the IANA is defined by
 a standards-track RFC, and such a definition includes:
        (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;

Klensin, Freed, Rose, Stefferud & Crocker [Page 4] RFC 1425 SMTP Service Extensions February 1993

        (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; and,
        (6)  how support for the extension affects the behavior of a
             server and client SMTP.
 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 an 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.
 Additional verbs are bound by the same rules as EHLO keywords;
 specifically, verbs begining with "X" are local extensions that may
 not be standardized and verbs not beginning with "X" must always be
 registered.

4.2. 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.

4.3. 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.

Klensin, Freed, Rose, Stefferud & Crocker [Page 5] RFC 1425 SMTP Service Extensions February 1993

4.4. 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.

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)>
                         ; 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

Klensin, Freed, Rose, Stefferud & Crocker [Page 6] RFC 1425 SMTP Service Extensions February 1993

 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].
        (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

Klensin, Freed, Rose, Stefferud & Crocker [Page 7] RFC 1425 SMTP Service Extensions February 1993

             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 non-standard 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 RSET to reset the connection, and,
             after getting a successful reply, send a HELO command and
             proceed as specified in RFC 821.

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.

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.

Klensin, Freed, Rose, Stefferud & Crocker [Page 8] RFC 1425 SMTP Service Extensions February 1993

 Miller, 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 1341, Bellcore, Innosoft, June 1992.
 [4] Moore, K., "Representation of Non-ASCII Text in Internet Message
     Headers", RFC 1342, University of Tennessee, June 1992.
 [5] Braden, R., "Requirements for Internet Hosts - Application and
     Support", STD 3, RFC 1123, USC/Information Sciences Institute,
     October 1989.

12. Chair, Editor, and Authors' Addresses

     John Klensin, WG Chair
     United Nations University
     PO Box 500, Charles Street Station
     Boston, MA 02114-0500  USA
     Phone: +1 617 227 8747
     Fax: +1 617 491 6266
     Email: klensin@infoods.unu.edu
     Ned Freed, Editor
     Innosoft International, Inc.
     250 West First Street, Suite 240
     Claremont, CA 91711  USA
     Phone: +1 909 624 7907
     Fax: +1 909 621 5319
     Email: ned@innosoft.com

Klensin, Freed, Rose, Stefferud & Crocker [Page 9] RFC 1425 SMTP Service Extensions February 1993

     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
     David H. Crocker
     The Branch Office
     USA
     Email: dcrocker@mordor.stanford.edu

Klensin, Freed, Rose, Stefferud & Crocker [Page 10]

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