Lookup for "554 5.2.3"
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 (5.2.3)
Message length exceeds administrative limit
A per-mailbox administrative message length limit has been exceeded. This status code should be used when the per-mailbox message length limit is less than the general system limit. This code should be used as a permanent failure.