I have found that this is the easiest combination that allows you to serve Virtual Domains, and Users. With this it's also very easy to implement webmail systems like SquirrelMail.
It allows completly crypted mail transport, remailer capabilities, maintainence via signed mail, etc. It's main propose is to enable political groups to communicate in private via mail.
The original disposable email service. Regain power over your inbox from commercial forces, and catch them spamming.
deny mail to unknown vhost-users