One or more third-party mail administrators report that they are not able to deliver mail to a domain that is protected by Email Security.cloud. The PTR record for the IP address cited by the third party mail administrator does not contain the words "messagelabs.com".
The third-party Message Transfer Agent (MTA) contains error messages such as:
Unable to connect to IP_address
...where IP_address is a valid IPv4 address with a PTR record that does not include "messagelabs.com". Further, this IP address is not part of the range of servers listed in the Firewall lockdown information in the document "Cloud security services IP ranges", available here: IP address ranges for Symantec.cloud
The third-party MTA attempted to deliver mail messages to an IP address that it not administered by Symantec Email Security.cloud.
Ask that the third-party mail administrator change the configuration of the third-party MTA so that it delivers messages using the MX records of your domain, rather than seeking to bypass the Anti-Spam and Anti-Virus security features of Email Security.cloud.