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SMTP Service Extensions

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 1869.
Authors Dave Crocker , Dr. John C. Klensin , Einar A. Stefferud , Dr. Marshall T. Rose , Ned Freed
Last updated 2013-03-02 (Latest revision 1995-05-08)
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IESG IESG state Became RFC 1869 (Internet Standard)
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Network Working Group                   John Klensin, WG Chair
Internet Draft                               Ned Freed, Editor
<draft-ietf-smtpext-extensions-04.txt>           Marshall Rose
                                               Einar Stefferud
                                                 David Crocker

                   SMTP Service Extensions

                         May 6, 1995

                     Status of this Memo

This document is an Internet-Draft.  Internet-Drafts are
working documents of the Internet Engineering Task Force
(IETF), its areas, and its working groups.   Note that other
groups may also distribute working documents as Internet-

Internet-Drafts are draft documents valid for a maximum of six
months. Internet-Drafts may be updated, replaced, or obsoleted
by other documents at any time.  It is not appropriate to use
Internet-Drafts as reference material or to cite them other
than as a "working draft" or "work in progress".

To learn the current status of any Internet-Draft, please
check the 1id-abstracts.txt listing contained in the
Internet-Drafts Shadow Directories on (US East
Coast), (Europe), (US West Coast),
or (Pacific Rim).

This draft is intended to supercede RFC 1651.

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.

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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.

 (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

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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).

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

4.1.  Changes to RFC 821

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

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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],

     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

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.

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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

     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

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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.

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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.

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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 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

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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

                       ; 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.

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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 SMTP service ready
       C: EHLO
       S: 250 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 SMTP service ready
       C: EHLO
       S: 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:

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       S: <wait for connection on TCP port 25>
       C: <open connection to server>
       S: 220 SMTP service ready
       C: EHLO
       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.

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

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but to include an entirely different solution from the one
originally proposed.

11.  References

[1]  J.B. Postel.  Simple Mail Transfer Protocol.  Request for
     Comments 821, (August, 1982).

[2]  D.H. Crocker.  Standard for the Format of ARPA Internet
     Text Messages.  Request for Comments 822, (August, 1982).

[3]  N.S. Borenstein, N. Freed.  Multipurpose Internet Mail
     Extensions.  Request for Comments 1521, (September,

[4]  K. Moore.  Representation of Non-ASCII Text in Internet
     Message Headers.  Request for Comments 1522, (September,

[5]  R.T. Braden.  Requirements for Internet Hosts -
     Application and Support.  Request for Comments 1123,
     (October, 1989).

12.  Chair, Editor, and Author Addresses

John Klensin, WG Chair
2100 Reston Parkway
Reston, VA 22091
 tel: +1 703 715-7361           fax: +1 703 715-7436

Ned Freed, Editor
Innosoft International, Inc.
1050 East Garvey Avenue South
West Covina, CA 91790
 tel: +1 818 919 3600           fax: +1 818 919 3614

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Marshall T. Rose
Dover Beach Consulting, Inc.
420 Whisman Court
Moutain View, CA  94043-2186
 tel: +1 415 968 1052           fax: +1 415 968 2510

Einar A. Stefferud
Network Management Associates, Inc.
17301 Drey Lane
Huntington Beach, CA, 92647-5615
 tel: +1 714 842 3711           fax: +1 714 848 2091

Dave Crocker
Brandenburg Consulting
675 Spruce Dr.
Sunnyvale, CA 94086 USA
 tel: +1 408 246 8253           fax: +1 408 249 6205

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