Best content-filter for Postfix

[Patric Falinder]

Hi,

I don't know if this is a valid Postifx question but I'll give it a shot.
What content-filter (spamfilter) does people use that has a large userbase?
Let's say I have over 1000 mailboxes which need to be checked for spam 
and 2 Postfix servers that will perform this task, what system is the 
most efficient for Postifx?

Thanks,
-Patric