There is one more e-mail app called "TypeApp".
https://itunes.apple.com/us/app/typeapp-email-mail-imap-exchange/id979735370?mt=8
Is it any good?
It's ok but lacks quite a few features (at least the last time when I checked) in comparison to the other apps which were mentioned earlier. See also my comparison here. Btw Bluemail = same as Type app.
Also check this critical blog about security on Type/Blue.