Track and trace displays an error message "421 4.4.0 [internal] no mail servers for this domain could be reached at this time" as well as IP 0.0.0.0 in the view logs section.
Delivered Status: Retrying delivery error 421 4.4.0 [internal] no mail servers
for this domain could be reached at this time
Symantec Email Security.cloud Service
The Track and Trace logs show an error stating "421 4.4.0 [internal] no mail servers 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.
As there was no response received from the recipient's MTA, our infrastructure inserts an internal error message. As the response from the recipient MTA was the equivalent of an IP of nulI, the IP of 0.0.0.0 is added and displayed in the logs. This behavior is by design to report an unsuccessful connection state to the remote host.
Please review and allow our Symantec Email Security.cloud services IP ranges on port 25 to ensure mail delivery to your infrastructure.
Note that the "MXs" part in error message "421 4.4.0 [internal] no mail servers for this domain could be reached at this time" does not refer to MX records in context but rather stands for "Mail Exchanger".
Symantec employs an exponential retry schedule based on a retry interval of 400 seconds until the interval exceeds the Max_Retry_Interval config setting (which is currently 12 hours - 43200 seconds).
Delivery ----- Seconds ------ D-HH:MM:SS
Attempt
1 ----- ----- 400 ----- --- 0-00:06:40
2 ----- ----- 800 ----- --- 0-00:13:20
3 ----- ----- 1600 --- ---- 0-00:26:40
4 ----- ----- 3200 --- ---- 0-00:53:20
5 ----- ----- 6400 --- ---- 0-01:46:40
6 ----- ----- 12800 - ----- 0-03:33:20
7 ----- ----- 25600 - ----- 0-07:06:40
8 ----- ----- 51200 - ----- 0-14:13:20
After the 8th try, it will attempt to deliver the message every 12 hours, until the email is delivered or expired. Messages expire once 7 days of delivery attempts have elapsed.