Problem in sending emails to gmail

Question

We are having issues with certain emails being sent to a distribution list in our O365 org are not being received in GMail. The error is

Reason: [{LED=421-4.7.28 Gmail has detected this message exceeded its quota for sending 421-4.7.28 messages with the same Message-ID. To best protect our users, the 421-4.7.28 message has been temporarily rejected. For more information, go to 421 4.7.28 https://support.google.com/mail/answer/188131 00721157ae682-708ae1fa64bsi. OutboundProxyTargetIP: 2607:f8b0:4002:c10::1b. OutboundProxyTargetHostName: gmail-smtp-in.l.google.com

Solutions described in certain forum are recommending setting up DKIM. I followed the instruction in https://learn.microsoft.com/en-us/defender-office-365/email-authentication-dkim-configure?view=o365-worldwide#steps-to-manually-set-up-dkim

I inserted a couple of CNAME records in our DNS zone but it seems like it's still not working (verified via nslookup).

Answer 1

Hi Stephen Carino,

To resolve the issue where emails sent to your O365 distribution list are being rejected by Gmail due to duplicate Message-IDs, follow these steps:

Address the Message-ID Duplication Issue

Root Cause: Gmail blocks messages with identical Message-ID headers sent in bulk (common with distribution lists). Office 365 typically generates unique Message-ID values, but misconfigurations or third-party tools might override this.

Verify Message-ID Uniqueness:

• Perform a Message Trace in the Microsoft 365 Admin Center.
• Check the headers of rejected emails (use Get-MessageTrace -RecipientAddress user@example.com in PowerShell or inspect headers via Outlook).
• Ensure each recipient receives a unique Message-ID.

Solutions:

• Avoid Custom Message-ID Generators: If using third-party tools or scripts to send emails, ensure they don’t hardcode the Message-ID.
• Test with Smaller Lists: Send emails to a small subset of the distribution list to confirm if the issue persists.
• Contact Microsoft Support: If O365 isn’t generating unique IDs for distribution list emails, escalate to Microsoft.

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