SpamTitan

SpamTitan provided a lot of the functionality of our previous mail filtering service, while keeping costs competitive. It's very simple and easy to use from an end user perspective, and not terribly hard to set up as an Administrator. There are plenty of options for deployments both small and large. On the back end SpamTitan uses a collection of open source software like Spamassasin and ClamAV, bundled together in a pretty web interface they've designed and provide support for. Compared to rolling your own solution, hiring a Linux admin to manage a mail filtering service, this is much more cost effective.

Support was a little lacking. Their headquarters are in Ireland, so responses take a little longer, and it's a little hard understanding them sometimes, especially over the phone. The audio quality of overseas phone calls is very poor.
The foundation SpamTitan is built on Spamassasin is only so good, it doesn't catch spam and viruses like the leading mail providers (Gmail/Google Apps). For that reason, it may require a little more time for administrators to fine tune settings.

Cloud based software so nothing to install on site. The dashboard allows you to see what emails have been quarantined, allows white listing of domains. Decent set of basic features. Price point reflects what you get.

The following lets it down:
1. The quarantine reports to the users are once a day, i get a lot of emails from staff saying they haven't received a email from a client and i have to log in to check, a quarantine email once a hour would be much better.
2. Some email which don't even match basic standards get block but not reported, took us a while to realise this as we had a ioT device misconfigured but had no idea Spam Titan was blocking it.
3. It can fail to deliver to the endpoint mail server, but doesn't report whether the email was successfully delivered or not just that it passed or didn't pass the spam listing, you have to go into the actual email and look at the headers.
4. Would be nice if there was a encrypted SMTP relay too that could scan email going out. Haven't had a compromised laptop or email account but could see how this would help.