Message Not Delivered: User authentication failed
The message could not be delivered because the sender's email program did not provide the sender's correct username and password to the sender's email provider email system, which requires these to prove the sender's identity.
In the sender's email program's account settings, look for "SMTP" (which is about sending email) and, in the SMTP area, "authentication". Ensure that the username and password are entered correctly there.
If this error persists, contact the email provider for help.
5.7.8: Official Definition
A permanent failure is one which is not likely to be resolved by resending the message in the current form. Some change to the message or the destination must be made for successful delivery.
This response to the AUTH command indicates that the authentication failed due to invalid or insufficient authentication credentials. In this case, the client SHOULD ask the user to supply new credentials (such as by presenting a password dialog box).