Postini alternative

Dear List Admins,

This is a question for some of the expert spam-fighters out there.
As you might imagine, The Mail Archive is on the  receiving
end of a fair amount of email spam. We currently reduce some
of that inbound spam using a service called Postini Message
Filtering. Unfortunately that service is being phased out so
so we are looking at alternatives. Does anyone have a particular
favorite amongst the spam filtering services?

-Jeff

http://postini-transition.googleapps.com/