Delayed Message: Authentication required
The email message has not yet been 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.
4.7.12: Official Definition
A persistent transient failure is one in which the message as sent is valid, but persistence of some temporary condition has caused abandonment or delay of attempts to send the message. If this code accompanies a delivery failure report, sending in the future may be successful.
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.