The full error message is below:
#< #5.7.133 smtp;550 5.7.133 RESOLVER.RST.SenderNotAuthenticatedForGroup; authentication required; Delivery restriction check failed because the sender was not authenticated when sending to this group> #SMTP#
In this case, an email was sent to a distribution group and the problem is that the distribution group was created with default settings which means it doesn't receive email from external senders. Yes, you read that right. You can even see it in a picture. See below:
To allow senders inside and outside your organization to send email to that group, check the box “Senders inside and outside of my organization” then click save.
You can also make this change using PowerShell as below:
Set-DistributionGroup Finance -RequireSenderAuthenticationEnabled $false
Now you should be able to send to this group.