Clement

If you're looking for an effective email server firewall, Clement might just be the solution you need. This application is designed to trap unwanted emails (a.k.a. SPAM) as early as possible, freeing up your email server to focus on its job of dispatching real emails without bouncing false originator emails.

Clement can work as a standalone tool, leveraging 'clamav' to detect viruses and 'spamassassin' to check for spam contents. Any detected viruses are rejected outright, with the virus name sent back to the real SMTP originator rather than a forged address. Meanwhile, spam is stored within a quarantine area in the system.

But that's not all Clement can offer. You can connect this program to a remote 'Mentor', which performs an in-depth analysis of the email context, assessing the origin trusting level and communication pattern of the intended recipient, and giving advice accordingly. With this additional feature, Clement is becoming smarter and more efficient, extracting real emails out of the clutter of messages that users receive every day.

The best part is that the Clement code is distributed under GPL license, which means that both binary and source code can be downloaded for free - an attractive proposition for those looking to save on software costs. Overall, Clement is a powerful tool that can help businesses and individuals to streamline their email environments, and it's definitely worth checking out.