xmspam is a site-wide XMail filter which calls spamc (SpamAssassin client) and adds the X-Spam headers in the message file.

xmspam will just add the headers - no mail will be dropped.

This is a site-wide implementation. If you need more features and you like C#, the code is very simple and easily customizable.

I haven't tested xmspam under Windows, but I suppose it should work.

xmspam is licensed under GNU GPL v.2