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We can delete the account and add it back. Open Mail application. Click on Mail menu and select Preferences. Under Accounts, select your BtInternet account and delete it by pressing the -
icon. Then restart your Mail. Again, goto Mail menu and select Preferences. Under Accounts, click +
icon. From the resulting window, select Add Other Mail account. Click Continue. Enter your fullname, full email address and password. Click Next. Use the following settings if asked.
Incoming Mail (POP3) Server - mail.btinternet.com
Port - 110
Outgoing Mail (SMTP) Server - mail.btinternet.com
Port - 25
Account Name/Login Name - Your email address (including the "@btinternet.com" or "@btopenworld.com" part)
Password - Your BT password
Please let me know if that didn't work, so we can continue troubleshooting.