I also have this problem since BT “super improved” my email last week.
I cannot send or receive.
They said that server settings were to be “Mail.btinternet.com” but they were that anyway.
I keep getting the message that username or password is incorrect, bit I know that they are ok as I can send and receive via “My BT”.
I tried deleting one of my accounts from my iPad and my iPhone, and now I cannot re-instate it.
I have posted the problem on the BT Community, but have still to receive a reply.
This is so irritating as it has worked ok for years, and now they have “super improved” it !!
I suspect that this is a BT problem, rather than an Apple problem.