Lookup for "451 4.7.12"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (451)
Requested action aborted: local error in processing
Enhanced Status Code Details (4.7.12)
Password transition required
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.