Topic on Manual talk:UserMailer.php

According to http://slopjong.de/2013/02/05/setting-return-path-in-php-mail/ I can set the Return-Path header with the 5th mail() parameter set to "-f xxx@yyyy.zz".

Is it possible to set it with this class?

There is a special hook for that, see Manual:Hooks/UserMailerChangeReturnPath!

Another option is the modify it in the AlternateUserMailer hook, where it is inside the $header variable as $headers['Return-Path'] . Ayway, the UserMailerChangeReturnPath hook was added especially for adjusting the return path, so I'd recommend using that one!