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Postfix Documentation
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XFORWARD Command syntax

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

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

The client may send the XFORWARD request at any time except in the middle of a mail delivery transaction (i.e. between MAIL and DOT). The command may be pipelined when the server supports ESMTP command pipelining.

The syntax of XFORWARD 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.

xforward-command = XFORWARD 1*( SP attribute-name"="attribute-value )

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

attribute-value = xtext

  • Attribute values are xtext encoded as per RFC 1891.

  • The NAME attribute specifies the up-stream hostname, or [UNAVAILABLE] when the information is unavailable. The hostname may be a non-DNS hostname.

  • The ADDR attribute specifies the up-stream network address, or [UNAVAILABLE] when the information is unavailable. Address information is not enclosed with []. The address may be a non-IP address.

  • The PROTO attribute specifies the mail protocol for receiving mail from the up-stream host. This may be an SMTP or non-SMTP protocol name of up to 64 characters, or [UNAVAILABLE] when the information is unavailable.

  • The HELO attribute specifies the hostname that the up-stream host announced itself with (not necessarily via the SMTP HELO command), or [UNAVAILABLE] when the information is unavailable. The hostname may be a non-DNS hostname.

  • The SOURCE attribute specifies LOCAL when the message was received from a source that is local with respect to the up-stream host (for example, the message originated from the up-stream host itself), REMOTE for all other mail, or [UNAVAILABLE] when the information is unavailable. The down-stream MTA may decide to enable features such as header munging or address qualification with mail from local sources but not other sources.

Note 1: an attribute-value element must not be longer than 255 characters (specific attributes may impose shorter lengths). After xtext decoding, attribute values must not contain control characters, non-ASCII characters, whitespace, or other characters that are special in message headers.

Note 2: DNS hostnames can be up to 255 characters long. The XFORWARD client implementation must not send XFORWARD commands that exceed the 512 character limit for SMTP commands.

Note 3: [UNAVAILABLE] may be specified in upper case, lower case or mixed case.

Note 4: the XFORWARD server implementation must not mix information from the current SMTP session with forwarded information from an up-stream session.

Note 5: 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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