I can view BT Yahoo mail on BT web page but if I try and download it to Thunderbird OR iPhone mail app I get a message " Sending of username did not succeed. mail server mail.btinternet.com responded invalid user name or password" I was able to download mail earlier this morning and nothing has happened to the computer since. I've rebooted twice. WIndows 10
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It is probably a BT server problem and the best advice when it happens is to do nothing, especially do not change passwords or delete the account from your phone or T'bird as that can cause more problems than it solves.
The first thing to do, which you have already done, when it happens is check to see if you can log on to your email account via a web browser. If you can there is obviously nothing wrong with your password or account hence the advice to do nothing. The problem usually resolves its self in a day or two.