Lookup for "550 4.1.4"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (550)
Requested action not taken: mailbox unavailable [E.g., mailbox not found, no access]
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.