Allow modifying the return-path of routed emails to allow better support of email auto responders.
Right now if you receive an email that you are forwarding with a route to gmail, Mailgun sets the Return-Path header to a bounce email. This does not play nicely with vacation reponder systems (namely Gmail). Those systems respond to the Return-Path and the auto-response doesn't go to the intended recipient. I'd like the ability to override the modified Return-Path and set it to the from/reply-to email.