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FRUSTRATION WITH EMAIL BOUNCE BACKS!!!!

Anonymous
2025-07-02T17:04:33+00:00

Context

For the last two weeks every message my team sends to any Google-hosted domain ( @gmail.com, G-Suite, Google Workspace, etc.) is bounced outright.  This includes long-standing 1-to-1 threads with customers and even messages to my own personal Gmail address.

Symptom Value
SMTP response 550 5.7.1 – “Gmail has detected that this message is likely suspicious due to the very low reputation of the sending domain. Message blocked.”
Auth results SPF = pass · DKIM = pass · DMARC = pass
SCL 1 (not spam in Exchange Online)
Recent changes NONE – no list mailings, no new marketing campaigns, no DNS edits

Repro steps

  1. Compose a plain-text or simple HTML email in Outlook 365.
  2. Send to any @gmail.com or other domain served by mx.google.com.
  3. Message is rejected within seconds – no delivery to spam, just a hard NDR.

Troubleshooting already done

  1. SPF / DKIM / DMARC re-validated with dmarcian + MXToolbox – all green.
  2. Checked sending IP against 80+ RBLs – clean.
  3. Created Gmail Postmaster account – domain reputation shows “Low” with no spam errors or volume spikes.
  4. Volume: < 150 total messages/week (transactional, person-to-person).
  5. Added List-Unsubscribe headers as a test – behaviour unchanged.
  6. Tried both Outlook desktop and OWA – identical result.

Sample bounce excerpt

550 5.7.1 [2a01:111:f403:2406::72a 19] Gmail has detected that this

message is likely suspicious due to the very low reputation of the sending

domain. To best protect our users from spam, the message has been blocked.

(…)

ARC / DKIM / SPF = pass · dmarc=pass action=none header.from=XXXXXXXX****

What I need

  1. Confirmation that Microsoft 365 isn’t missing some new Google-specific header or policy (one-click unsubscribe, etc.) that would silently tank reputation.
  2. Any Office 365 transport / connector tweaks that have solved similar reputation blocks.
  3. Guidance on expediting reputation reset – Postmaster data is sparse and Google support refers us back to our mail provider (i.e., Microsoft).

This issue is crippling client communications and began without any configuration change on our side.  Any insight or escalation path inside Microsoft would be greatly appreciated.

Thanks in advance,

Didier

Microsoft 365 and Office | Subscription, account, billing | For business | Other

Locked Question. This question was migrated from the Microsoft Support Community. You can vote on whether it's helpful, but you can't add comments or replies or follow the question.

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  1. Anonymous
    2025-07-02T19:38:30+00:00

    Dear Didier Leonard-Jean Charles,

    Thank you so much for your prompt follow-up and for sharing your experience. I truly appreciate your kind words and your recognition of my previous response; it means a great deal.

    I’m pleased to hear that the issue has been resolved and that your test emails are now reaching Gmail accounts successfully. It’s great that you were able to identify the rule affecting delivery and take action to correct it.

    If you found my reply helpful and the explanation clarified the redirection behavior, I kindly invite you to share your feedback using the feedback tool.

    Many users rely on the Microsoft Support Community to find solutions, and your input can help others with similar questions discover accurate and timely information more easily.

    Thank you again for your patience and understanding throughout this process. If there’s anything else I can assist you with, please don’t hesitate to reach out.

    Warm regards,

    Vy-Ng - MSFT | Microsoft Community Support Specialist

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  2. Anonymous
    2025-07-02T18:53:29+00:00

    Thank you for the response. I was able to resolve the issue with the help from Microsoft support. It looks like one of my rules was causing the issue, which is extremely confusing (and surprising) to me, considering the rule simply added legal disclaimer text to the bottom of my email - nonetheless, I deleted the rule and sent a few test emails to GMAIL accts and they all went through

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  3. Anonymous
    2025-07-02T18:43:52+00:00

    Dear Didier Leonard-Jean Charles, 

    Good day! Thank you for reaching out to the Microsoft Support Community and sharing your experience. I understand how important it is for your team to maintain reliable communication with clients, and I appreciate the time you've taken to outline the situation. 

    To help us move forward effectively, I’d like to confirm a few details and offer some initial steps: 

    1. Did you recently add your custom domain to your Microsoft 365 tenant? 
      New domains may require time to build reputation with external providers like Gmail, especially if email volume is low or inconsistent
    2. Is DKIM configured for your custom domain (e.g., yourcompany.com)? 
      Microsoft 365 uses a default DKIM signature unless custom DKIM is manually set up. This alignment is important for Gmail’s reputation scoring. 
      Setup guide: How to use DKIM for email in your custom domain - Microsoft Defender for Office 365 | Microsoft Learn
    3. Are there any transport rules or connectors in place that bypass spam filtering? 
      These can unintentionally affect domain reputation. If you're unsure, I’d be happy to help review your configuration.
    4. Have you already opened a support request with Microsoft? 
      If not, I recommend submitting a sample message and bounce-back details here: 
      Manage submissions - Microsoft Defender for Office 365 | Microsoft Learn

    What We Know So Far 

    • Microsoft 365 includes all required headers for Gmail. Optional headers like List-Unsubscribe and ARC are supported and can help improve deliverability.
    • Gmail’s domain reputation scoring is based on authentication, engagement, and historical behavior. Even with correct SPF, DKIM, and DMARC, low reputation can still result in message rejection.
    • Gmail Postmaster Tools do not offer a manual reset, but consistent sending behavior and reduced bounce rates can help improve reputation over time.

    Once I have your confirmation on the above, I’ll be glad to assist with next steps or help escalate the issue further.  

    Warm regards, 

    Vy-Ng - MSFT | Microsoft Community Support Specialist

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