RFC 2505: Anti-Spam Recommendations for SMTP MTAs

Recommendations on how to setup an SMTP server to make it less likely to be abused by spammers. The most important points are that you should use IP-based rules for relaying and provide verbose Received-lines and log as much as you can.

However, it is also mentioned that implementing these recommendations won't stop spam as anyone can still connect directly to each recipient host, instead of using an open relay. But at least the spammer will use more of their own bandwidth and if the recipient host have these rules implemented, it'll be easy to track him down.