MDaemon is more close to postmaster needs than MS solution.

Besides the fully-featured mail-server solution, it has (no order intended):

- Bayesian spam filter / learning engine
- integrated web-mail possibilities
- full support for SSL encryption for secured access to: web / POP3 / SMTP / IMAP access
- very powerful account management system: alias, quota, permissions etc
- ODBC support for accounts / lists
- instant messaging (YES!)
- content filter - well done!
- simple log / config / accounts system: NO registry, NO system integration - you can basically MOVE an entire server in a matter of MINUTES to another machine and make it RUN! (you just have to COPY and, in bad situations, to edit path in config file!
- excellent ANTIVIRUS support (powered by KAV engine), with just-in-time update system - NO virus has ever passed through several servers since I started to use this software! (years!!!)
- a lot of SECURITY settings that you can control in-deep - and very reliable on this side, too!
- LDAP, GroupWare full support!
- Bandwidth throttling
- mobile client devices support

Newer versions have also more features, but hope I'll be back sometime soon with another review! ;)

PS: Just ONE MAJOR inconvenient: It’s NOT cheap! Not at all!