I encountered this one recently. Google doesn’t make it simple (or clear) how to allow external people to send emails to a Google Group (which most people use as the equivalent of a distribution list / Office365 group).
In Google Admin, navigate to the Google Group and check off “Allow” (a checkmark) where Publish Posts and External intersect (seen below with the number 1 in red)
That’s it! External users will now be able to email the group.