How can the Return-Path header be different than the actual email bounce recipient?

Question

I recently moved my transactional email sending to Mailgun

It works good so far however I am wondering about the return-path header.

Consider this email (I removed

Answer 1

Turns out that there are different kinds of bounces.

When bounces occur they are generally returned to the server that is sending the email, and do not follow the MX records.

Thats why they are sent to the mailgun servers and also arrive there.

However there are also so called "Delayed Bounces" that are sent to the server declared as mailserver using MX records in the domain.

Those delayed bounces are generally difficult to handle and there are opinions out there that they violate RFC.

Those bounces are however very, very rare. Thats why mailgun does not handle them. The reason they use the clients domain in the return-path address is so that they can assign it to the right account. They just encode it that way...

In fact, as I was setting up my mailbox for bounces on my google apps mail, I recieved one such delayed bounce.

It was this email that made proper debugging possible which lead to the understanding of this issue.

So to sum up:

Yes, the address is incorrect. That is no problem for most bounces since the server does not use MX records to send them, but sends them directly to the server that has initated the conneciton.

However in case of delayed bounces, that also some times happen, the bounce will indeed go to the server behind the mx records of the domain specified in the return path address.

Those emails are not properly recognised as bounces at mailgun servers.