The full text of this error message looks something like this:
The message could not be sent because the server rejected the sender’s email address. The sender’s email 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
To resolve “address rejected” error messages
Enable the SMTP server authentication feature in your email client.
Note: Every email client has different instructions for enabling this feature. You can find instructions for your email client in the client’s online help, or by searching the Internet.
https://www.att.com/support/smallbusiness/article/smb-email-support/KM1189242/