The FSS (ForeFront Server Security) team blogged about antispam features and Exchange Server, really interesting post.
Microsoft Antigen, which protects your Exchange 2000 and Exchange 2003 servers, provides robust anti-spam capabilities. On these servers, your Anti-Spam settings will be found in the Antigen UI, not in the Exchange UI. Just keep in mind that if you want to take advantage of the Anti-Spam features of Antigen, you will need to download Antigen for Exchange with Antigen Spam Manager from the Messaging Security Suite. If you do not download this version, when you open the Antigen Administrator you will see the Anti-Spam icon display for a second and then disappear.
If you are running Exchange Server 2007, you will find that Microsoft Forefront Security does not provide Anti-Spam features. Why is Spam filtering included in Antigen, but not in Forefront, you ask? That is because Anti-Spam protection is provided by Exchange 2007, so it would be redundant to include these features in Forefront as well. Although Spam filtering has moved from Forefront to Exchange, Forefront does still provide keyword filtering, which identifies unwanted e-mail messages by analyzing the contents of the message body. By creating keyword lists, messages can be filtered based on a variety of words, phrases, and/or sentences.
Read the full story at source: http://blogs.technet.com/fss/archive/2007/05/23/a-funny-thing-happened-on-the-way-to-the-technet-forum.aspx