4.1.1.1. Extended HELLO (EHLO) or HELLO (HELO)

These commands are used to identify the SMTP client to the SMTP server. The argument clause contains the fully-qualified domain name of the SMTP client, if one is available. In situations in which the SMTP client system does not have a meaningful domain name (e.g., when its address is dynamically allocated and no reverse mapping record is available), the client SHOULD send an address literal (see Section 4.1.3).

RFC 2821, and some earlier informal practices, encouraged following the literal by information that would help to identify the client system. That convention was not widely supported, and many SMTP servers considered it an error. In the interest of interoperability, it is probably wise for servers to be prepared for this string to occur, but SMTP clients SHOULD NOT send it.

The SMTP server identifies itself to the SMTP client in the connection greeting reply and in the response to this command.

A client SMTP SHOULD start an SMTP session by issuing the EHLO command. If the SMTP server supports the SMTP service extensions, it will give a successful response, a failure response, or an error response. If the SMTP server, in violation of this specification, does not support any SMTP service extensions, it will generate an error response. Older client SMTP systems MAY, as discussed above, use HELO (as specified in RFC 821) instead of EHLO, and servers MUST support the HELO command and reply properly to it. In any event, a client MUST issue HELO or EHLO before starting a mail transaction.

These commands, and a "250 OK" reply to one of them, confirm that both the SMTP client and the SMTP server are in the initial state, that is, there is no transaction in progress and all state tables and buffers are cleared.

Syntax:

ehlo = "EHLO" SP ( Domain / address-literal ) CRLF

helo = "HELO" SP Domain CRLF

Normally, the response to EHLO will be a multiline reply. Each line of the response contains a keyword and, optionally, one or more parameters. Following the normal syntax for multiline replies, these keywords follow the code (250) and a hyphen for all but the last line, and the code and a space for the last line. The syntax for a positive response, using the ABNF notation and terminal symbols of RFC 5234 [7], is:

ehlo-ok-rsp = ( "250" SP Domain [ SP ehlo-greet ] CRLF )

/ ( "250-" Domain [ SP ehlo-greet ] CRLF

*( "250-" ehlo-line CRLF )

"250" SP ehlo-line CRLF )

ehlo-greet = 1*(%d0-9 / %d11-12 / %d14-127)

; string of any characters other than CR or LF

ehlo-line = ehlo-keyword *( SP ehlo-param )

ehlo-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")

; additional syntax of ehlo-params depends on

; ehlo-keyword

ehlo-param = 1*(%d33-126)

; any CHAR excluding <SP> and all

; control characters (US-ASCII 0-31 and 127

; inclusive)

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 specified in RFC 821 and Section 2.4.

The EHLO response MUST contain keywords (and associated parameters if required) for all commands not listed as "required" in Section 4.5.1 excepting only private-use commands as described in Section 4.1.5. Private-use commands MAY be listed.