Don't Be a Spam Source:
Your first duty in the spam war is to make sure that your system is not a source of spam. Your system can be a source of locally generated spam, or it can be a relay for spam generated elsewhere. You need to respond to both possibilities.
Define an Acceptable Use Policy
To prevent locally generated spam, you need to make sure that everyone using your server knows
that sending unsolicited advertisements from your server is not allowed. I have to admit that the
idea of such a thing is alien to me. The government agencies and large businesses that I work with
would fire anyone who misused corporate property in any way, let alone anyone running a private
advertising firm on a corporate server!
But your situation may be different. You may be offering a community service on your system. In that case, you need a written Acceptable Use Policy (AUP) that tells people what type of use is allowed and what isn't. If you're not sure what an AUP should look like, ask your ISP, or check a national ISP. All large ISPs have some form of AUP.