Lookup for "554 5.6.1"
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 (5.6.1)
Media not supported
The media of the message is not supported by either the delivery protocol or the next system in the forwarding path. This is useful only as a permanent error.