You set the distribution group to receive emails only from specific senders in the delivery management section of the DL properties. Now an email from external source sent to this DL and it fails with the NDR with the following error message:

#550 5.7.1 RESOLVER.RST.NotAuthorized; not authorized ##

Error message says it can not resolve the external email address. The solution is very simple.

Create an Exchange mail contact for the external email address that sends emails to the distribution group.

And, add the Exchange contact to the allowed senders list in delivery management section. That’s all.