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:

• the textual NAME of the SMTP service extension;
• the EHLO keyword value associated with the extension;
• the syntax and possible values of parameters associated with the EHLO keyword value;
• 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);
• any new parameters the extension associates with the MAIL or RCPT verbs;
• a description of how support for the extension affects the behavior of a server and client SMTP; and,
• 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.

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:

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.