Hi,
since June we are subscribers of
http://www.messagelabs.com. Before I also tried to fight spam on my own, but I gave up, as it was too time-consuming. As Messagelabs is a managed service, they do all the processing and the admin efforts at our end are minimal. At the beginning you have to fill some addresses of newsletters in a whitelist, as newsletters are often falsely identified as spam. We are a small company, so Messagelabs were the only service we could use since their competitors that I asked for quotes are targeted and priced for big companies with 1000 seats and more. Their smallest package is for 25 users.
Regards,
Frank