The ability to send a different email as an attachment. This is an Outlook feature that I miss.
I'm not so sure that I'd want to have emails selectable by a file manager. It's hard enough to keep track of emails in the app. When people ask for this, do they really mean to have email message files mixed in a folder with other files, or do you want to present the emails to a file manager as if they were files so we can organize them more efficiently with the file manager and so Kostya does not have to give AquaMail all the features of a file manager?