I you are still considering I would like to add Kerio Mail server to your options
http://www.kerio.com/kms_home.html
Kerio MailServer is a super-fast secure multiple domain mail server working with all POP3 and IMAP desktop mail clients for Windows, Linux and Mac.
Kerio MailServer™ represents a new generation of mail servers designed for corporate networks. To help combat increasing security threats, Kerio MailServer™ offers a wide range of features to keep email from being intercepted, infected by computer viruses, or sent as spam.
It does have a price tag on it (http://www.kerio.com/kms_price.html).
It has a trial edition (test it before you buy it).
http://www.kerio.com/kms_download.html
Kerio MailServer runs on Windows 2000/XP/2003, server and desktop editions, Red Hat Linux, SUSE Linux and Mac OS X. Kerio Outlook Connector for Microsoft Outlook runs on Windows 2000/XP/2003
Cheerios
noodle