Install Xeams on Debian
Xeams is a secure and powerful mail server for Windows, Linux, Solaris, MacOSX and other flavors of UNIX, and supports SMTP, POP3, and IMAP. Xeams includes a powerful spam filtering engine that eliminates up to 99% of junk email right out of the box. The software offers a flexible approach to email, making it a very user friendly server.
According to the Xeams site and I agree with them.
It's easy, fast and highly customizable.
I use my Xeams in a HA (master-slave) setup between two sites with AD integration and 10 AD user profiles.
It scans my in and outbound and even encrypts my messages between mail servers that accept it.