I have been using Messagelabs at work for about a year and it does a good job. It has been taken over by Symantic recently and is still doing a good job of removing spam, I check it most days but have not found any false positive - I work for a medicine manufacturer and have not found my orders in with the Viagra!
Clapton - I realise I should be asking them questions and only an idiot would hand over money and appoint them without speaking to them face to face but I wanted un-biased reviews from people already using their services like the pleasant straight forward un-sarcastic respone wiz-king has given ( thank you wiz-king )
wiz-king - are messagelabs not expensive? mailfilteruk.com seem to do the same thing but alot more cost-effective
Messagelabs charged us £775 for 20 e-mail addresses filtered for spam, viruses and other nasties for a year. As we get about 400 e-mails a day its not to bad - saves me a lot of time but I do miss the occasional funny e-mail telling me I have won the lottery.