In early 2004, I was looking for a solution of reducing spam email or our domain. We had been using SpamAsassin, but we didn't have a food soluton for training the bayesian database.
Then I found Maia Mailguard. This is a extension of amavisd-new. When mail is received it is run though spam an virus filters. The mail is quarantined or released based on each users settings. There is a nice web interface for users to mark items as spam, that were missed, or release items that were incorrectly marked as spam.
I installed this on my server, and it really worked for our mail. It worked so well, that I decided to make this available to other people. That is when I started MailBarricade. I mentioned this to my friends, and posted some ads with AdWords. Some friends and some small companies signed up.