There seems to be quite a lot of similar questions but I haven't found one which covers my situation.
I have an android device on loan to do some development testing and at some point I opened the standard Mail account and connected to my Gmail account. Obviously all my mail was synced.
Now I need to return the device and I have set Aeroplane mode to avoid any further access to my WiFi.
However, when I run the Mail app all my previously synced emails are there, obviously stored somewhere on the device.
I need a way of deleting all those emails from the device only. Maybe deleting a file somewhere on the device or something else ?