I have worked as a software developer for large corporations for several decades, my users are happy users.
There's an old saying... "you pay peanuts, you get monkeys". I'm not saying that is the case here, I have no idea what BT pay their software teams.
I have tried adding both "@mydomain" and "myemail@mydomain" to the safe senders list under settings and BT (mx.bt.lon5.cpcloud.co.uk) is till rejecting valid emails sent to me!
Perhaps it is time to set up that gmail account.