When I was in need of such we used a service such as "electric mail company." I purposely omitted the link.
We rarely saw spam. I'm sure there are other companies like that.
The question is what is the best solution to replace Postini by example.
Many products propose Hosted Anti spam or Cloud Based solutions.
Depending of the number of mailbox to protect and also the possibility to manage your filtering criteria, one product can be better then another one.
What are your real experiments regarding these solutions?