Three months ago, I installed the NoSpamToday! SMTP Proxy on my dedicated server box (you can read about the adventures encountered in my blog entry Web applications and SMTP proxies don't mix well). Today I had a look at the statistics:
On average, the proxy rejects four out of five mails before they reach the mail server - for reasons ranging from malformed headers, banned file extensions, virus-contaminated attachments, and a SpamAssassin-based spam detection. Needless to say that my inbox is virtually spam-free since then. Neato.