Definition and Registration of Extensions

The IANA maintains a registry of SMTP service extensions. A

corresponding EHLO keyword value is associated with each extension.

Each service extension registered with the IANA must be defined in a

formal Standards-Track or IESG-approved Experimental protocol

document. The definition must include:

o the textual name of the SMTP service extension;

o the EHLO keyword value associated with the extension;

o the syntax and possible values of parameters associated with the

EHLO keyword value;

o 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);

o any new parameters the extension associates with the MAIL or RCPT

verbs;

o a description of how support for the extension affects the

behavior of a server and client SMTP; and

o the increment by which the extension is increasing the maximum

length of the commands MAIL and/or RCPT, over that specified in

this Standard.

In addition, any EHLO keyword value starting with an upper or lower

case "X" refers to a local SMTP service extension used exclusively

through bilateral agreement. Keywords beginning with "X" MUST NOT be

used in a registered service extension. Conversely, 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.

Additional verbs and parameter names are bound by the same rules as

EHLO keywords; specifically, verbs beginning with "X" are local

extensions that may not be registered or standardized. Conversely,

verbs not beginning with "X" must always be registered.