Lookup for "550 2.4.2"
This status code indicates a successful delivery or other operation. Any details are simply notes.
Basic Status Code Details (550)
Requested action not taken: mailbox unavailable [E.g., mailbox not found, no access]
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.