Lookup for "551 4.7.12"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (551)
User not local; please try
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.