Some emails remain in the delivery queue and show a 451 error in the delivery queue view of the Control Center Message Queues view.
Emails remain in the delivery queue with an SMTP error code of one of:
451 4.4.2 [internal] no banner
451 4.4.2 [internal] no BODY response
451 4.4.2 [internal] no RSET response
The Symantec Messaging Gateway (SMG) connects to the downstream server successfully, but does not receive a response. SMG considers this a successful connection with a transient failure in delivery and will continue to attempt to send to the problematic MX record.
nslookup -type=mx gmail.com