Track and trace displays an error message "421 4.4.0 [internal] no MXs for this domain could be reached at this time" as well as IP 0.0.0.0 in the view logs section.
Symantec Email Security.cloud Service
Delivered Status: Retrying delivery error 421 4.4.0 [internal] no MXs for this domain could be reached at this time
The track and trace logs would show error "421 4.4.0 [internal] no MXs for this domain could be reached at this time" and IP address as "0.0.0.0" . The combination of these 2 items indicate that the Symantec MTA could not successfully connect to the remote host.
Since there was not a response by the recipient's MTA, our infrastructure inserts an internal error message, as well as adding the IP 0.0.0.0 to display in the logs as the response from the recipient MTA was equal to null. This behavior is by design to report an unsuccessful connection state to the remote host.
Possible factors that may cause this issue
Connection refused by remote MTA
Connection times out while trying to connect to the remote MTA due to a network security device such as an IPS, Traffic Shaper device or Firewall (packet inspection).
Firewall rule blocking connection from local MTA IP address
A device on the internetwork causing the connection failure on the network path