When an external sender attempts to send an email message to an Office 365 group they may receive a non-delivery report (NDR).
The NDR states:
Your message couldn’t be delivered because the group you’re sending to needs to know who you are before it will accept your message. To fix this problem, as the email admin for the group to configure the group to accept messages from you.
There is additional information for email administrators:
This error occurs when the group is configured to reject email from senders outside of the group’s organization.
There is additional diagnostic info with a specific NDR status code:
Remote Server returned ‘550 5.7.133 RESOLVER.RST.SenderNotAuthenticatedForGroup; authentication required; Delivery restriction check failed because the sender was not authenticated when sending to this group’
This type of NDR is usually caused by the setting on groups in Office 365 that controls whether people outside the organization can send email to the group. When a group is first created in Exchange Online, whether it is created manually or by a cutover/staged migration, the setting defaults to “Off”. This setting is visible in the Office 365 admin portal, in the Groups section.
Switch the setting to “On” to enable external senders to send email to the group.
PS C:\> Set-DistributionGroup teamalpha -RequireSenderAuthenticationEnabled $false