Hello, I work for a local Government center. We currently have MDaemon installed as a switch between our MS Exchange server and the Internet. Why? I have not found any product for the price and ease of use. It has filters for emails and now incorporates SpamAssassin into Mdaemon. Blacklists and whitelists, IP ban lists antivirus solution, oh yea, ease of use and configuration. Built in listserver, SSL, Mobile device support. If you get the Groupware plugin you can even use Outlook, not just outlook express, so you have the calendar to use also.
No, I don't work for them if you are wondering. I just think it is a great product. Download the demos and give them a try and see, I doubt you will be disappointed.