I really like SpamTitan's web user interface, it's clear, easy to understand, and the service was easy to set up. I love the licensing system. We have a lot of emails account, but most of them are not so much used.
The licensing system is calculated about last days average email count. it's great for us.
Now we have a full filtering system for all incoming emails, and the final users (mailbox) can easily delete, release or whitelist email from a daily report.