Any one who knows me, knows I have fought the pressure to put in Microsoft Exchange at our company for years. I have been a user of ALT-N's Mdaemon product now for years. Currently we are using their 9.x version. I have always been extremely happy with MDaemon for the following reasons:
1. I think it delivers Enterprise level email server services at an excellent price
2. It has very good security with SPAM/Antivirus/Bayesing/White and Black Lists, etc.
3. LDAP support if desired
5. Web Access for users
6. IM secure for users
7. Multiple Domain Support
8. Gateway support
9. Mailing Lists, Alias, Forwards, etc.
10.Excellent Upgrade process AND ... it runs great in a Virtual Server
Those are just a few of the reasons I like it. Some are part of the Pro Version, so check it out.
But now ALT-N is giving a 5 user version at no cost! Now that is cool, giving that for years I paid for their 5 user version to run my own email server at home! So, I say this is a great move on their part. See below, from an email sent to me to upgrade to their version 10:
 INTRODUCING MDAEMON FREE *
"MDaemon FREE" is a limited but totally free version of MDaemon. It is
intended to provide basic mail server capability for up to 5 users. For
more information on MDaemon FREE visit: http://www.altn.com/MDaemon-Free/
The installer will offer an option to install MDaemon FREE or you can
convert an existing installation or trial version to MDaemon FREE by using
this registration key: IDJDFYC-YQDSBOL-HTXANTQ
Now this is cool!