Hello everyone (and especially you, Kostya
)
I answer this topic quite old because, nowadays in 2019, Aquamail remains one of the only applications that can not write on an external SD card (and yes, I know it's not a file manager; D!) ... in any case, I did not find the way to save the configuration on a directory of the external SD card
All the -very numerous- apps I'm using on Android are using différents "files providers" or "files pickers" that let me choose the directory by the use of my favorite FileManager (for exemple : X-Plorer).
Can't you consider to make this evol for this fisrt class piece of software... please ?
Best regards