Hosted Spam Filter for POP3

I’m looking for a hosted Spam Service to utilize for a SMB. This company has offices in several location and has their email hosted through a third party. The intent is that any incoming mail would hit the pop3 host, the spamhost would retrieve email from the pop3 host and the user would retrieve processed mail from the spamhost.

I have found a couple of possible solutions, SpamArrest and BopSpam. Does anyone have experience with these services or have you used another service that you could recommend?

Thanks