mailscanner

http://mailscanner.info/
 ClamAV
 SpamAssassin

esvacommunity

http://www.esvacommunity.com/

ESVA (E-Mail Security Virtual Appliance) is a pre-built and semi-configured email scanning appliance that will run on VMware Workstation, Server, Player or ESX Server.

p3scan

http://p3scan.sourceforge.net/                     # no ui

You have to set up a port redirection with iptables so that all connections from e.g. inside your office to any POP3/POP3S/SMTP server outside in the world will not leave your router, but come to a local port, on which P3Scan listens.

scrolloutf1

http://www.scrolloutf1.com/
OS: Debian
Minimum Requirements:
384 MB of RAM
6 GB storage

 Web GUI address: http://host-ip/
 Web GUI User: Admin
 Web GUI pass: 123456

Whitelist [sender domain] using drag & drop emails (with MS Outlook)
Blacklist [sender address] using drag & drop emails (with MS Outlook)
Assign an outbound IP address per domain
Relay outgoing messages through a global or per sender domain ISP (smarthost)
Disclaimer per domain