Message Not Delivered: Authentication required
The message could not be delivered because the message requires a password for authentication.
In the sender's email program's account settings, look for options about authentication of outgoing messages. Make sure that the password entered is accurate.
If this error persists, contact the email provider for help.
5.7.12: 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 user needs to transition to the selected authentication mechanism. This is typically done by authenticating once using the [PLAIN] authentication mechanism. The selected mechanism SHOULD then work for authentications in subsequent sessions.