Lookup for "501 2.4.2"
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 (2.4.2)
Bad connection
The outbound connection was established, but was unable to complete the message transaction, either because of time-out, or inadequate connection quality. This is useful only as a persistent transient error.