Found a two part article from IBM developer works on how to control spam mails using advanced SMTP settings in Lotus Domino. Written by Edmond Stanton, the article illustrates how to control spam mails by using the Configuration Settings document, server mail rules, and inbound SMTP commands and extensions in Lotus Domino 6 and above.

Should be an interesting read for Domino administrators facing problems with mailbox getting filled with spam mails. Part1 and Part 2 of the article titled Controlling spam: Advanced SMTP settings in Lotus Domino can be found in developer works dated 4th Oct 2004.