Hope its not a glitch on their end. Lets check it again. Delete the account from Mail.
Then delete the com.apple.mail.plist file.
Open Mail, click File > Add Account. Setup your account.
When setting up, if asked for Incoming Server details, use the following settings;
- Select 'POP' as the Account Type.
- Make sure Incoming Mail Server is set to mail.btinternet.com
- Make sure your User Name (the same as your email address) is correct.
In the Incoming Mail Security window, leave the box next to 'Use Secure Sockets Layer (SSL)' unchecked.
On the Outgoing Mail Server window, make sure the Outgoing Mail Server is set as mail.btinternet.com
and the box next to 'Use Authentication' should be ticked.
The User Name and Password XXXXX your BT Yahoo! email address and password.
In the Outgoing Mail Security window, leave the box next to 'Use Secure Sockets Layer (SSL)' unchecked.
If you are still getting the error, then we can try Mail in a different test account. That should help us to isolate the problem