Lookup for "504 4.2.3"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (504)
Command parameter not implemented
Enhanced Status Code Details (4.2.3)
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.