When email is sent to a domain protected by Symantec Email Security.cloud, the service responds to the SMTP RCPT TO: command with the error, "421 Service Temporarily Unavailable".
421 Service Temporarily Unavailable
The sending server is hitting the secondary MX preference of the Symantec.cloud customer, such as clusterXa.xx.messagelabs.com.
This causes connections to be deferred with this error.
Email should hit the primary Symantec.cloud MX preference, clusterX.xx.messagelabs.com; legitimate servers should never connect to the backup MX, clusterXa.xx.messagelabs.com.
Note: We provide 100% redundancy on the primary MX, which has multiple mail servers behind it.
See Redirecting inbound email traffic to Email Security.cloud to learn how to set up MX records with the correct preference.
Once the administrator of the sending server takes action to prevent spam from being sent through their IP address, Symantec is willing to reset the IP reputation.
To resolve this, flush the server's DNS cache by running the Windows command: