Lookup for "554 5.6.2"
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 (554)
Transaction failed
Enhanced Status Code Details (5.6.2)
Conversion required and prohibited
The content of the message must be converted before it can be delivered and such conversion is not permitted. Such prohibitions may be the expression of the sender in the message itself or the policy of the sending host.