Exchange 2007 has a built in anti-spam filtering module that just needs to be enabled.
Here are some simple instructions to get going with it. Once you’ve enabled this feature, you should see an anti-spam tab in the Exchange management console under Organization configuration > Hub Transport > Anti-Spam.
To enable anti-spam functionality on a Hub Transport server
1. Open the Exchange Management Shell
2. cd to the scripts folder in the Exchange install directory.
%system drive%/Program Files\Microsoft\Exchange Server\Scripts folder.
3. Run the following command:
4. After the script has run, restart the Microsoft Exchange Transport service by running the following command:
5. Enable automatic anti-spam updates.
Forefront Anti-spam Automatic Updates is a premium feature that requires either an Exchange Enterprise client access license (CAL) for each user mailbox, or a Forefront Security for Exchange Server license.
You can see how to configure automatic updates here.
Some info for this article was taken from these Microsoft articles.