I like SpamTitan's ability to manage multiple domains, and set up multiple account administrators, also the option to send users daily spam filter digest messages. TitanHQ also offer self-hosted versions for those with specific security needs.
I'd give SpamTitan a shot for sure, they only charge you for active mailboxes, so pricing is much more reasonable than even the online pricing would indicate. For example, one of our clients has a mail server with about 150 currently "active" (user provisioned) mailboxes, however many of them get little to no use, or are only used for calendaring, meetings rooms, etc, and so our actually monthly licenses usage count for this customer is only 90 seats.
Our business needed a managed cloud based solution for multiple tenants. This allows us to manage Spam filtering for multiple clients, each running different types of e-mail services, all through a single, cloud hosted and managed, pane of glass.