Exchange 5.5 + Spam Filtering Software

I have a client using Exchnage 5.5 who desperately needs ome spam filtering software to run on her exchange server. What wouold people recommend. I have seen Surfcontrol, McAfee spam killer, Policy Patrol Mail Essentials, the list is endless. Something nice and easy, that will aloow mail to get through. Some I have seen block out mail that you would want.
Thanks in advance
Shaun