Lookup for "553 5.1.5"
Permanent Error - This status code indicates a permanent issue and is unlikely to resolve itself. You should contact the Email Service Provider.
Basic Status Code Details (553)
Requested action not taken: mailbox name not allowed [E.g., mailbox syntax incorrect]
Enhanced Status Code Details (5.1.5)
Destination address valid
This mailbox address as specified was valid. This status code should be used for positive delivery reports.