I just changed Return-Path in 2 of my servers which run Exim and Posfix. The third server runs Sendmail. I spent several hours trying to find information how to change it but can not find anything. So, I need to change Return-Path so that sent emails contain Return-Path: firstname.lastname@example.org instead of email@example.com.
I can not change the server's hostname itself because my data center wants additional 5 EUR/month for a custom PTR record.