Spamassassin for Outlook (as a plugin) seems to be an inactive project:
There's a plugin for Outlook that works on a per-user basis to learn what is and what is not spam - but it requires some manual training:
If you want to host your own anti-spam and antivirus mail filter for multiple users, in house, I can strongly recommend an Open Source app called MailScanner, which runs on a Linux-based server. MailScanner wraps SpamAssassin, ClamAV, and a number of other optional (free and non-free) antivirus and anti-spam tools around an MTA ('mail server'). The learning curve can be a bit steep if you have no Linux experience but the mailing list members are a great bunch and are very helpful.