Spam control under Post. Internet News Internet Investing Internet Technology Windows Internet Tech. There's not much you can without putting something in front of the mail server.
Some people like to put sendmail in front for spam control then send it off to post.office.
Or there is mailshield which can either run on the same server or on a seperate server which is specifically designed for spam filtering. Office seems very weak in it's ability to block illegitimate messages. Spam control under Post.