We have a setup with Redmine where users often send mail using different from addresses in the email. We use Google Apps for email, but this would apply to other mail systems where users can set up multiple from addresses in their email.

If users send email with a different from address than their registered one then Redmine ignores the incoming email.

To fix this issue I created the attached modification which checks the mail headers "Return-Path", "Sender" and finally "From" (as before) for a match to a user account.

The "Return-Path" header relies on getting mail server to set this to the envelope sender address. I use the preline program with qmail to get this set.

This works because Google Apps email (and GMail) always set the Sender and envelope sender addresses to the users main or canonical email address even if they choose a different "From" address.