The SMTP Server app uses Postfix (http://www.postfix.org/) in either of two modes - SMTP server or mail gateway.
As an SMTP server, it can be used as an on premise mail server, providing email to multiple custom domains. It can be a cost-effective and secure solution to alternative cloud-based mail services like GoogleApps or hosted-MS Exchange. No per-user fees in addition to a number of plug-in app extensions that provide antivirus, antimalware and spam scanning, greylisting, archiving and more.
Alternatively, this app can be configured as a pass-through filter, scanning email as it passes through to an existing mail server on your network. The most popular configuration in this mode is to filter mail before passing on to an MS Exchange server - preventing malware via email and spam from ever reaching the the Exchange server and users inboxes.
To learn more about this app, click here
Documentation for ClearOS 6 can be found here.
Documentation for ClearOS 7 can be found here.