Lookup for "451 5.7.12"
Permanent Error - This status code indicates a permanent issue and is unlikely to resolve itself. You should contact the Email Service Provider.
Basic Status Code Details (451)
Requested action aborted: local error in processing
Enhanced Status Code Details (5.7.12)
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.