This error message means that you need to enable SMTP server authentication in the email client you are using. The full error message will probably say:
"The message could not be sent because the server rejected the sender's e-mail address. The sender's e-mail address was (Email address). Subject (Subject), Account: (Account name), Server: (Server name), Protocol: SMTP, Server Response: '505 Authentication required', Port: 25, Secure(SSL): No, Server Error: 505, Error Number: 0x800CCC78"
The bold part of this error message tells you that this feature is not enabled. To learn how to set up your email account and enable SMTP server authentication in different email programs, use the link below:
If your program is not listed on the above link, you may still be able to enable SMTP authentication. However, we cannot offer instructions for setting this up on all versions of all email programs. If your email program does not support authenticated SMTP you may have trouble using it for BT Yahoo Mail.
If you're having problems with your emails or you're receiving error messages, please use the link below for further information about error messages and how to resolve them: