Unable to receive inbound emails from external distribution list. I

Hi Shauna Lucas

Thank you for reaching out to Microsoft Q&A

Based on your description, you are unable to receive inbound emails sent from an external distribution list in Exchange Online. You have already tried setting the SCL to -1, adding the sender to safe senders, checking blocked domains, and verifying that messages are not in quarantine or message trace.

As far as I know, this problem typically occurs when the distribution group settings in Exchange Online restrict external senders or when message headers such as Return-Path are misconfigured. Additionally, in some cases, authentication requirements, delivery management settings, or SPF/DKIM alignment can cause external systems to reject or drop messages. Additionally, anti-spam policies may block messages with null senders, which is common for distribution lists.

From my perspective view, you can try the below instructions to see if the issue can be resolved

1. Verify delivery management settings: In Exchange Admin Center, ensure the distribution group allows messages from outside the organization and that “Require that all senders are authenticated” is unchecked.
2. Update report to parameters: Use PowerShell to enable ReportToOriginatorEnabled or ReportToManagerEnabled so that messages have a valid Return-Path:

      Set-DistributionGroup "<GroupName>" -ReportToOriginatorEnabled $true
   

3. Check DNS and Authentication: Confirm your domain’s MX record points to Microsoft 365 and that SPF includes include:spf.protection.outlook.com. Enable DKIM and DMARC if not already configured.
4. Review Anti-Spam policies: If null sender blocking is enabled, adjust policies or rely on the ReportTo settings above to prevent message rejection.

Hope my answer will help you, for any further concern, kindly let me know in the comment section

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