Network Working Group J. Klensin, WG Chair

Request for Comments: 1651 MCI

Obsoletes: 1425 N. Freed, Editor

Category: Standards Track Innosoft

M. Rose

Dover Beach Consulting, Inc.

E. Stefferud

Network Management Associates, Inc.

D. Crocker

Silicon Graphics, Inc.

July 1994

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. Standard 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.

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RFC 1651 SMTP Service Extensions July 1994

(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 STD 11, 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).

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RFC 1651 SMTP Service Extensions July 1994

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

STD 10, 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.

4.2. 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.

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:

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RFC 1651 SMTP Service Extensions July 1994

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;

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RFC 1651 SMTP Service Extensions July 1994

(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 a standard or standards-track SMTP

service extension registered with IANA. A conforming server must not

offer non "X" prefixed keyword values that are not described in a

registered and standardized 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.

4.4. 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.

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RFC 1651 SMTP Service Extensions July 1994

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

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

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RFC 1651 SMTP Service Extensions July 1994

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

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 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

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>

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RFC 1651 SMTP Service Extensions July 1994

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.

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, 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.

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RFC 1651 SMTP Service Extensions July 1994

[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 Authors' Addresses

John Klensin, WG Chair

MCI Data Services Division

2100 Reston Parkway, 6th floor

Reston, VA 22091

USA

Phone:: 1 703 715 7361

Fax: +1 703 715 7435

EMail: klensin@mci.net

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

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RFC 1651 SMTP Service Extensions July 1994

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

Silicon Graphics, Inc.

2011 N. Shoreline Blvd.

P.O. Box 7311

Mountain View, CA 94039

USA

Phone: +1 415 390 1804

Fax: +1 415 962 8404

EMail: dcrocker@sgi.com

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