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Postfix Documentation
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XCLIENT Command syntax

An example client-server conversation is given at the end of this document.

In SMTP server EHLO replies, the keyword associated with this extension is XCLIENT. It is followed by the names of the attributes that the XCLIENT implementation supports.

The XCLIENT command may be sent at any time, except in the middle of a mail delivery transaction (i.e. between MAIL and DOT, or MAIL and RSET). The XCLIENT command may be pipelined when the server supports ESMTP command pipelining. To avoid triggering spamware detectors, the command should be sent at the end of a command group.

The syntax of XCLIENT requests is described below. Upper case and quoted strings specify terminals, lowercase strings specify meta terminals, and SP is whitespace. Although command and attribute names are shown in upper case, they are in fact case insensitive.

xclient-command = XCLIENT 1*( SP attribute-name"="attribute-value )

attribute-name = ( NAME | ADDR | PROTO | HELO )

attribute-value = xtext

  • Attribute values are xtext encoded as per RFC 1891.

  • The NAME attribute specifies an SMTP client hostname (not an SMTP client address), [UNAVAILABLE] when client hostname lookup failed due to a permanent error, or [TEMPUNAVAIL] when the lookup error condition was transient.

  • The ADDR attribute specifies an SMTP client numerical IPv4 network address, an IPv6 address prefixed with IPV6:, or [UNAVAILABLE] when the address information is unavailable. Address information is not enclosed with [].

  • The PROTO attribute specifies either SMTP or ESMTP.

  • The HELO attribute specifies an SMTP HELO parameter value, or the value [UNAVAILABLE] when the information is unavailable.

Note 1: syntactically valid NAME and HELO attribute-value elements can be up to 255 characters long. The client must not send XCLIENT commands that exceed the 512 character limit for SMTP commands. To avoid exceeding the limit the client should send the information in multiple XCLIENT commands; for example, send NAME and ADDR first, then HELO and PROTO.

Note 2: [UNAVAILABLE], [TEMPUNAVAIL] and IPV6: may be specified in upper case, lower case or mixed case.

Note 3: Postfix implementations prior to version 2.3 do not xtext encode attribute values. Servers that wish to interoperate with these older implementations should be prepared to receive unencoded information.

Postfix Documentation
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