http://www.gentoo.org/doc/en/virt-mail-howto.xml

For most gentoo users, a simple mail client and fetchmail will do. However, if you're hosting a domain with your system, you'll need a full blown MTA (Mail Transfer Agent). And if you're hosting multiple domains, then you'll definitely need something more robust to handle all of the email for your users. This system was designed to be an elegant solution to that problem.