We use mxguarddog for our spam filtering, it's cloud based, very cheap and does the job with very few false positives, redirect your MX records to their servers, set up users and you're good to go, doesn't sync/integrate with AD or exchange though which would be nice.
Guys, the filtering services you are using at the moment, i'm just curious, but do any of them double as a proper contingent mail store in the event that a primary exchange server goes down, i.e how long does the service retain mail, and can users access the email directly via a web portal in the event of an emergency?
To the OP what has GFI Mail Essentials been like for you cons/pros wise? I was looking at it a while back, but never got around to trying it. I've used the GFI Vipre AV and found it to be satisfactory enough at keeping malware out of a clean system, but absolutely awful at cleansing systems post infection, in fact i've lost a mail store due to it's 'remedial' actions in the past and i've never forgiven it since, so i'm little dubious as to GFI products to be honest.