Kostya,
First let me say, Best Android E-mail app bar none.
However, one thing I have found myself occasionaly wishing for, that "MAY" be relatively easy to implement, is a "SMART PURGE". I say "MAY" because I know how programming can be.
What I mean by SMART PURGE, is that when you are in the "SMART Folder" and have sent items to their respective deleted folders, it would be nice to have a single "Purge all deleted from all accounts" option in the same way that you right now can use purge all on an individual account's deleted folder. It could be made as an option on the Smart Menu that appears if any account has something in it's deleted folder.
What do you think?
Fizix