I know that this issue has already been raised but I would also like a advice on how to avoid these annoying banners which regularly appear on the BT Mail app on both my own and my wife's smartphone. We both have Android phones and they are fully updated.
The easy way to avoid them is not to use the BTMail app. There are many different email apps for the Android system all of which will work with BTMail.
I use the Aquamail app paid version and it works very well and has no adverts or banners.