Lookup for "501 2.2.3"
This status code indicates a successful delivery or other operation. Any details are simply notes.
Basic Status Code Details (501)
Syntax error in parameters or arguments
Enhanced Status Code Details (2.2.3)
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.