failure to accept username of imported email address

I have successfully imported messages from one of three BTinternet addresses but ‘server’ continually says username or password of the other two I have entered are incorrect. They are definitely correct. How can I proceed?

Sometimes the password can be rejected if the settings are incorrect.

Go to Menu > Tools > Accounts and make sure you have the following combinations of port and policy.

SMTP Port 587 = Force usage of SSL/TLS
IMAP Port 993 = Use SSL/TLS on special port (legacy)
POP3 Port 995 = Use SSL/TLS on special port (legacy)