If you don’t have backpress you can also modify your bb_mail function (in bb-includes/pluggable.php)
just add the email you want as return-path.
before ::
$headers = trim(join(“rn”, $headers));
after::
$headers = trim(join(“rnReturn-Path: from@mydomain.comrn”, $headers));
It is not elegant but it works.
Remember it is hardcoded, you’ll have to change it manually if you want to change your from@mydomain.com later.
