Symantec Messaging Gateway (SMG) shows messages in the delivery queue with the message "421 4.4.0 [internal] no MXs for this domain could be reached at this time".
This is not an error but a description of why the message could not be delivered. The message indicates a general mail delivery attempt failure in which the appliance could not communicate with the remote mail system. This may be the result of an inability to successfully connect to the remote host, to resolve the MX records, or to resolve the DNS host names for the email domain to which the appliance is attempting message delivery. This can also be seen if a Control Center host's Quarantine SMTP listener is not available on port 41025.
Possible circumstances that may cause this issue:
nslookup -type=mx example.com
telnet -b [SMG's Outbound IP] [IP Address from step 1] 25