Anti Spam software for Exchange

I would assume you are using 2003. Use MS's Antigen for Exchange 2003. It works well.

If you are using Exchange 2007, I would use MS's Forefront Security for Exchange 2007.

Now if you want to use a third party, I would say GFI is decent, we use a lot of Trend Micro SMB stuff now. Since we no longer advise out clients to use Symantec EP...

Also, if you use IMF that's built into exchange 2003 SP2 and Exchange 2007, you'll get a decent amount of it caught.

Good luck!