I have clients in the medical field. They had the same issues with most of the spam software out there. They work in animal research.
I installed SpamDam at
http://www.spamdam.net
It works like a white list system that the user can manager there own list. The other problem with this client, is there server is a AS/400, and must of the server solutions are for Windows servers. SpamDam worked fine here for all there servers. They have AS/400, Windows, and Linux servers.
Since the user managers there own list, there is very little work load for the admin. Also any Spam message can be held so the user can look at it, if they want to.