Bluemail (not as nice as eM Client) allows me to select just one of my accounts at a time. This is very handy because it means I can’t then accidentally reply from the wrong account.
Ideally, have a panel with a vertical list so instantly jump from one to another and reply from that account (no mistakes). Or select Unified to have an overall view. (Maybe someone would like to select multiple but not all … )
Note that I have the free version and only a single email account in eM. This is because I have Outlook and Bluemail too and it simply keeps them separate. If you add the functionality I suggested, I might well BUY eM Client as I would then use it for several email accounts … (and escape from Bill Gates !)