Postfix XCLIENT Howto

Purpose of the XCLIENT extension to SMTP

The XCLIENT command targets the following problems:

1. Access control tests. SMTP server access rules are difficult to verify when decisions can be triggered only by remote clients. In order to facilitate access rule testing, an authorized SMTP client test program needs the ability to override the SMTP server's idea of the SMTP client hostname, network address, and other client information, for the entire duration of an SMTP session.

2. Client software that downloads mail from an up-stream mail server and injects it into a local MTA via SMTP. In order to take advantage of the local MTA's SMTP server access rules, the client software needs the ability to override the SMTP server's idea of the remote client name, client address and other information. Such information can typically be extracted from the up-stream mail server's Received: message header.

3. Post-filter access control and logging. With Internet->filter->MTA style content filter applications, the filter can be simplified if it can delegate decisions concerning mail relay and other access control to the MTA. This is especially useful when the filter acts as a transparent proxy for SMTP commands. This requires that the filter can override the MTA's idea of the SMTP client hostname, network address, and other information.