In some web servers that use the PHPMailer, Apache inserts in the Return-Path header of the emails, the email address in the PHP settings, but this action can provoke that notification emails with the download link be considered as spam. The current update, forces the Return-Path to the email address defined in the store's settings.

git-svn-id: https://plugins.svn.wordpress.org/music-store/trunk@1013885 b8457f37-d9ea-0310-8a92-e5e31aec5664