Lookup for "551 2.7.12"
This status code indicates a successful delivery or other operation. Any details are simply notes.
Basic Status Code Details (551)
User not local; please try
Enhanced Status Code Details (2.7.12)
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.