I have Magento using Sendmail and Wordpress using PHPmailer to send webapp-generated mail. Occasionally, someone will enter their email address incorrectly and the mail (let's say, a purchase receipt) will bounce back to the return-path specified by the script.

I dont want to set the return path for each vhost, especially because it is not easily done. Ideally, WP would use the address of the blog admin and Magento would use one of the numerous email fields specified, but they default to using username@machinename (in my case, username is the system user and machinename is a FQDN, but it is not the same as the actual vhost FQDN). The result is that bounced mail returns to the server and, since the server is used only for outbound SMTP, the messages sit there, undelivered and worse, unread.

I'm Postfix 2.6.6 on CentOS 6.3, is it possible to globally force a specific returnpath for all messages sent via PHP on the server?