Linux – How to set Return-Path header of postfix bounced messages

Go it!

Ok, so when your using fetchmail, you define the "alias" (local user name) in the fetchmail configuration (/etc/fetchmailrc) not in /etc/aliases. Use the smtpname directive.

Example:

set syslog;
set daemon 60;

poll "mail.server.com"
with protocol pop3
user "nrahl@mydomain" password "12345" is "nrahl@localhost"
fetchall
smtpname "nrahl@localhost"

Return-Path is the same thing as is used in the MAIL FROM: command in the SMTP dialogue. It is often, but not always, the same thing as the From:-header within the mail.

So when you use SMTP to send a mail, the address used in MAIL FROM must be the address to which you want any bounces sent. The From:-address would usually be the address you'd want a human to reply to.

There's also Reply-To, which may be a third address. This is commonly used e.g. for mailinglists - when the listserver sends out the messages, the MAIL FROM will be the email address for the listserver itself, so that it can handle any bounces. The From: will be the address of the author of the mail, and the Reply-To will be the lists's mailing address so that any response to the email will normally go to the whole list.