Compact folder/account navigation. I like my email client unclutered and eM client does that very well appart from the left navigation panel. It can be reduce but then it lost all its usability and buttons, its nice though to be able to set it back to its normal state with just a click on the double arrow.
would be a good idea to keep the double arrow even in extended mode so it can be reduced without having drag its border.
the best imho would be to have a very compact mode with just icons representing the folders/accounts.