M365 Account

Hi @Trott, Stephanie, 

I hope you’re having a great day. I appreciate your patience and the details you provided. 

A - What’s most likely happening 

From what you shared, this behavior typically indicates that the issue is specific to the group’s delivery settings rather than a tenant-wide mail flow problem. 

Microsoft 365 Groups include a setting that controls whether people outside your organization can email the group (commonly shown as “Allow external senders to email this group” or “Let people outside the organization email the group”). If this option is disabled even unintentionally external (unauthenticated) messages will be blocked, and external senders may receive a non-delivery report (NDR) such as 550 5.7.193, or the message may not be delivered at all. 

B - Recommended checks 

(If you're not an admin, please contact your organization's administrator to proceed these steps)  

1/ Confirm the Group allows external senders  

You can enable external delivery for the group in the Exchange admin center: 

• Please go to Microsoft Admin Center >  Exchange admin center (EAC) > Recipients > Groups 
• Select the Microsoft 365 Group > Settings > enable “Allow external senders to email this group” > Save  

Changes can take time to propagate (Microsoft notes up to ~1 hour in some cases).  

More info here: NDR 550 5.7.193 when external users send email to Microsoft 365 group - Exchange | Microsoft Learn 

2/ Confirm the Group allows external senders 

This can also be managed from the Microsoft 365 admin center: 

• Go to Teams & groups > Active teams & groups 
• Select the group > Settings > enable “Let people outside the organization email this group” > Save 

It may take up to ~30 minutes for external senders to be able to email the group after enabling this. 

More info here: microsoft-365-docs/microsoft-365/admin/create-groups/manage-groups.md at public · MicrosoftDocs/mic… 

3/ Alternative fix via PowerShell (if preferred) 

If you manage groups via Exchange Online PowerShell, Microsoft’s documented command is: 

Set-UnifiedGroup -Identity <group> -RequireSenderAuthenticationEnabled $False  

Additionally, in the meantime, if you need external mail to flow right away, you can temporarily have external senders email a shared mailbox (since those are working) and then forward/route internally as needed. 

C - Contact Support 

In case your IT department cannot access the necessary information or make the required changes, I recommend asking them to open a request with Microsoft Support team through the Microsoft Admin Center. They have access to backend configurations and can perform a more in-depth investigation. At the very least, they can provide the most effective workaround to ensure your experience remains smooth and secure.     

We kindly ask for your understanding that, as moderators, we do not have access to the administrative tools or permissions required to investigate or modify these configurations. We also do not have the capability to initiate remote sessions for direct assistance. Our role is to guide users to the appropriate resources and support channels.     

I hope this information is helpful. Should you have any further questions or need additional assistance, feel free to reach out.   

I look forward to hearing your thoughts on this. 

If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have any extra questions about this answer, please click "Comment".  

Note: Follow the steps in our documentation to enable email notifications if you want to receive email notifications related to this topic.