Vorpal Mail is intended as a more user-friendly (or should that be admin-friendly) replacement for the standard sendmail package. It's aims are to provide all the features that are required of a modern mail server, without all the complexity caused by sendmail's rules based configuration file.
vPostMaster is a Postfix mail server which integrates the best Open Source virus and spam filtering, and email server policy. Its features include: greylisting, SPF, per-user filtering rules, per-domain policy defaults, easy yum installation and configuration, spam and virus filtering, and can be administered through a web browser.
eXtended Account Managing Software: able to manage e-mail accounts for several domains and sites
Fast, rock stable, secure, GPL, feature filled and multiplatform ESMTP/POP3/FINGER/POPSYNC server
XMail Queue Manager
gives you full control over the multi level nested mail queue of the server. You can track messages in their different states (sending, resending, frozen). Statistics tell you about the number of spooled messages. Its crossplatform remote feature allows you to manage queues of servers without GUI.
Z1 SecureMail Gateway
(commercial) a central, server-based software solution which provides encryption and digital signatures for the entire email traffic of an organization