Lookup for "551 2.1.4"
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 (2.1.4)
Destination mailbox address ambiguous
The mailbox address as specified matches one or more recipients on the destination system. This may result if a heuristic address mapping algorithm is used to map the specified address to a local mailbox name.