This is what i got from the maker.
Open the app's settings, turn off "scheduled mail checking" and "IMAP
push". After this, you can force stop the app if you wish, and it
should not restart.
Those settings are always accessible, and as far as the app's memory
usage, if this is your concern --
Its background service does not "fight back" if Android needs to kill
it, due to low memory (some other apps do),
That is, unless you've enabled IMAP Push, in which case it will "fight
back" if killed -- but that's intended, so the app can provide instant
incoming mail notifications.
If you would like to keep scheduled mail checks and be sure the app
lets go of memory for other app in between -- just disable IMAP Push
and keep scheduled mail checks enabled.