Recently working with an organization they had a group in Exchange that wasn’t receiving emails. Their bounce back had a diagnostic code I used to research the issue.

Diagnostic-Code: smtp;550 5.7.1 RESOLVER.RST.AuthRequired; authentication required

This article help me solve this issue.

https://social.technet.microsoft.com/Forums/exchange/en-US/b7128bf2-4ec0-4508-840f-32a34a12ac8b/550-571-resolverrstauthrequired-authentication-required?forum=exchangesvradminlegacy

I hope this helps someone else.