AXIGEN Gateway is a secure mail relay server, available for Linux, FreeBSD, OpenBSD, NetBSD and Solaris. AXIGEN uses firewall-like connection rules and is capable of integrating with most virus / anti-spam applications. By separating security functions from real mail server usage, you improve both server effectiveness and communication security.
During the evaluation period and first year of use, users have free tech support.

SecExMail makes e-mail encryption as easy as sending a message. There are no plugins necessary and it will work with any standard email client including Microsoft Outlook, Outlook Express, Calypso, Eudora and many more. Now includes automatic key exchange and powerful html filtering to protect from malicious email. Personal Edition, including support can be purchased for $4.95 using PayPal or Credit Card. Commercial and Linux versions available.

Merak Mail Server provides unlimited accounts and domains for one price, which will save you costs as your needs grow. At anytime you can decide to add new modules and your data and settings will be preserved. Merak contains Integrated Antispam, Integrated Antivirus, Groupware Server & Calendaring for MS Outlook.