Add recommendation to use Return-Path module

Issue Summary

After I installed and tested the Mass Contact module I experienced the "return-path" fault that occurs when sending email via Drupal websites. This led me to the Return-Path module (see: http://drupal.org/project/returnpath) and I really think that all mail modules should recommend an installation of that module to circumvent sending autoreplies to complete strangers as it might happen on a multisite server.
(Why that module isn't built into core I'll never understand, but that's a different discussion).

HTH
/elfur