Some spammers learn that you use graylisting – which is great because they try to send less spam to your server. This is at least what I learnt since I have implemented graylisting on one of my mailservers. Besides the
Fail2ban
Fail2ban scans log files like /var/log/pwdfail or /var/log/apache/error_log and bans IP that makes too many password failures. It updates firewall rules to reject the IP address. Resources http://www.fail2ban.org/wiki/index.php/Main_Page
SpamAssasin Alternatives
If your tests show that you really can’t use SA, then other choices are dspam and bogofilter. but these are “pure” statistical filters. so you need to train them to get anything. Another choice is FUSSP – Final Ultimate Solution
iptables Video Tutorial Link
Mastering IPTables, Part I. http://www.linuxjournal.com/video/mastering-iptables-part-i
Is ASSP better than SpamAssasin?
As far as I read, ASSP is a great competitor to SpamAssasin. I will give it a chance on my test server and I will generate a report about it. ASSP with ClamAV (make sure you use the Sane Security