How to block external domains from sending mails to internal distribution lists

Hi All,

Good Day!

We have a Symantec Messaging Gateway, relaying inbound mails. There are a few distribution lists created at the Exchange level.

The requirement is that we do not want external domains to send mails to the DL. However individual members of that DL must continue to receive mails from external domains.

Is there a way to achieve this with the SMG. SMG is running software version 9.5.3-3.

Please assist. 

Regards,

Faiz