I use SpamAssassin, and than filter the emails marked Spam to a folder where I can go through them later or just let them accumulate than mass delete them. Unfortunately SA can't seem to get those emails that have no subject and no sender.
We tried Anti-Spam SMTP Proxy Server on UNIX servers. It was the most effective and lightest solution to block 98% of SPAM traffic, but the problem is that the daemon crashes itself after about 12 hours without logging any error; tried some scripts to monitor and start it automatically, but it never worked.
Using SpamAssassin currently, but it consumes a lot of CPU resources. It blocks most of the SPAM traffic, but we still receive a lot of spam
However, ASSP is working like a charm on Windows Servers