Is your email address is published on your website on Internet? This might be the reason for a lot of your spam.
Squid is a caching proxy software, licensed under GNU GPL, (free software).
It can help your Organization to reduce bandwidth and improves response times by caching and reusing frequently-requested web pages.
Squid has extensive access controls and if you don’t mind Command Line Interface is not too hard to configure. It runs on most available operating systems including Windows.
The following article is a step by step HOW TO Install Squid on Debian. Most of the steps are easily applicable to any other Linux distribution such as: RedHat, Suse, Ubuntu, etc…