One solution is to define the -f sendmail parameters in the php.ini 'sendmail_path'. This is no good when running a single Apache instance with PHP as there is only one php.ini and you need to specify different return addresses for each domain. This module resolves the problem by parsing out the return-path set by Drupal modules, and passing it to the sendmail binary using the -f option, rather than sending it only in the $headers parameter of the mail() command. - DrupalEasy