As regards what you were told about paying to keep your email address a few years back, that was correct back then but not for the last couple of years.
As regards your comments about Apple software on an Apple device, If you re-read what I posted I was referring to using software ie apps other than from the manufacturer of those devices using the devices operating systems ie software, not about hardware so it is relevant.
Given that you have had problems with the BTMail app for over a year I am surprised that you are sticking with it
Re: Unable to add file attachments to emails - BT Community
Having said that, as has been said, if your choice is to carry on using the BTMail email app just carry on using it.
Please post back to let us know when you finally get the BTMail app doing all the things that you want and what rival email apps already do.