$20 per month, per user seems a little high when compared to Gmail which is either free (personal) or $5/month (business). The fact that Google is able to charge for email is partially due to the fact that they offered it for free for so long and got so many people to like the experience that it created a lot of demand in the workplace for it - I think you really need a way to let people experience your application for free if you want any shot at enticing people to pay $20/month for it.

There may be some subset of people willing to pay $20/month for ultra-configurable filtering, but I'm not really sure how big that market is, and I doubt it will help you break into larger organizations.