Tere.
I highly recommend that anyone who wants a very lightweight, *very* effective, brain-dead simple (installation *and* maintenance) anti-spam gateway check out ASSP (http://assp.sourceforge.net/)... it was already one of the most effective options available, but there has been a *lot* of activity recently, and the improvements are amazing. Combined with ClamSMTP (http://memberwebs.com/nielsen/software/clamsmtp/) - also very light-weight, very easy to install, configure and maintain - it has reduced the load on my server drastically (through its early HELO checks, and the new PB (PenaltyBox) functionality, and is virtually maintenance free. Well, I haven't tested it yet, but did check the code, it's written in Perl, so surely it's not lightweight, and in Your case, maybe configuring the smtp do the HELO check, would improve the situation even without this software. And Assp is also old - June 26, 2005, can't actually trust that old piece of code...
Btw, I just tested ClamSmtp, and it passed true viruses, but Amavis did catch them.
-- Sysadmin