Does not play well with others

Returnpath tries to declare drupal_mail_wrapper even if it is already declared; this causes problems if there's more than one mail-processing-type module installed. See http://drupal.org/node/111690#comment-496295 - this node details an SMTP module but the error and the cause are the same.

Maybe Drupal needs some kind of "Incompatible With" module field to go with the "Dependent On" one.