Why doesn't my HTC M8 One delete emails or sync properly (BlueMail app)?

[AC Question]

HTC M8 One / BlueMail app / syncing problem

I am new to a HTC M8 One.
I didn't like the stock email app, and downloaded BlueMail app which I'm told is one of the best. I love everything about the app, except one thing...
It doesn't seem to delete emails or sync properly compared to other mail apps.

I am using an email address that uses Exchange.

any advice?