I have created an Office 365 group. I am the owner. It simply has 5 members and the goal is to allow customers to send an email to the address and have it distributed to the 5 members.
I have Accept messages from: All Senders selected and have Reject messages from: No Senders selected.
Despite those selections no email addresses from outside the group can send emails to the group. The following is bounced back to them:
|Reported error:||550 5.7.193 UnifiedGroupAgent; Delivery failed because the sender isn't a group member or external senders aren't permitted to send to this group.|