This old chestnut again.
Trying to retrieve email in Thunderbird, was working earlier this morning, and been working for years before.
"Sending of password did not succeed. Mail server mail.btinternet.com responded: Incorrect username or password."
Clicking OK gives.
"Login to server mail.btinternet.com failed."
This seem to happen periodically and often for a day or more.
Consensus seems to be it's a BT mail server problem but nobody ever seems to know why, or who to notify at BT to try to fix it.
Do you access this email account with more than device ?
Are you using POP or IMAP ?
No, just one device.
It's an intermittent problem that's been happening for years to numerous users but nobody seems to know what causes it.