Having sent an email to one particular domain an NDR comes back to the Symantec Messaging Gateway (SMG) with the error "501 5.7.1 This system is not configured to relay mail"
501 5.7.1 This system is not configured to relay mail
There is normally an issue with the mail server that is receiving the mail.
Have a look at the error message to determine where the message is coming from. If there is an internal relay step in the company that server will need to be ruled out as the source of the message.
Once the source has been determined then it is necessary to look up the admin of the problem mail server and then contact that person to help with a resolution.
It is possible to determine who the responsible admin is by using any number of tools such as the whois command or one of the many online IP/MX lookup tools that are freely available.
This problem is indicative of a problem or a setting on the receiving mail server normally and should have nothing to do with the SMG, contact that admin for help.