HOWTO: CiviMail Return Channel for Multi-site Servers
Setting up the CiviMail return channel is a notorious pain in the ass. The return channel is the mechanism by which email is passed back into the CiviMail system. It's necessary to allow replies and bounces to be handled by CiviCRM, making it fairly essential to the successful use of CiviMail.
The official way of setting things up is unfortunate in a couple of respects: it requires the installation of a customized version of the amavisd-new content-filtering daemon*, and it cannot handle multi-site installs. The "alternative" return channel implementation, although rumoured to not scale as well, requires no additional software, and can be easily adapted for servers hosting multiple domains that use CiviMail.
