I have tried a number of Mail apps and for the moment have setteled on Moxier (trial).

I have found many of the free apps to be lacking in features, and to have trouble conecting to my corporate servers.

One big feature for me is having a seperate password for the mail app, rather than having my phone password have to meet my BYOD restrictions.

I will give MD a try. I know some of the other "high end" mail clients are in the $20 range, but it still feels expensive. Moxier Pro and Touchdown are over priced.
Consider a more intermediate price to get batter pro take up.

It does developers diservice to loose potential customers and good word of mouth because you are trying to get $18 from 5%, rather than $10 from 20% of a much larger pool of users.

Good Luck,
Charles