These are all great and really helpful.
I have had this problem for a few months now on and off. I contacted BT Help today and they told me to delete the email account in my settings (Apple) and then add it again. It took a while with verifying very slow and it said it couldn’t load something use -msomething else instead (I said yes) and it seems to have worked.
I have also downloaded the BT Mail app so if it happens again I don’t have to log into my BT account just to send an email.
Whether this is a long term fix remains to be seen... But thanks for your help, Beverly