Lookup for "251 4.1.4"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (251)
User not local; will forward to
Enhanced Status Code Details (4.1.4)
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.