The Return-Path: header is intended to show the envelope address of the real sender as opposed to the sender used for replying (the From: and Reply-To: headers). In posting Usenet news, for example, the Return-Path: shows "news" and the From: shows the address of the posting user. But in general, Return-Path: should never be used for replying to mail. It is intended to be used solely for notification of delivery errors.

There must be only one Return-Path: header in any mail message, and it should be placed there by the site performing final delivery. This header should be declared in the configuration file like this:

H?P?Return-Path: $g

The ?P? flag ensures that only delivery agents that perform final delivery insert this header. Those delivery agents are usually prog and local, which usually contain an F=P delivery agent flag.

The $g macro ($g) contains as its value the address of the sender relative to the recipient.

Unfortunately, two circumstances can cause the Return-Path: header to contain incorrect information. First, the message might arrive at your site with this header already there. If this happens, that wrong header will normally not be replaced. You can, however, define H_ACHECK (Section 25.6.1.1) in conf.c and cause this header to be replaced even if it is already in the message.

The second problem stems from the fact that final delivery might not really be final. The local delivery agent program might be something such as procmail(8), which allows mail to appear to be locally delivered, while also allowing users to run shell scripts that can forward their mail to another site.

To minimize these problems, always declare the Return-Path: header with the proper ?flags? in the configuration file. Doing this ensures that it will be inserted when legal and that the address your site places in it is usually correct.