Why am I suddenly seeing an increase in bounces from Microsoft domains?

Key findings

• Sudden impact: Bounces from Microsoft (Outlook, Hotmail) can appear very suddenly, even for IPs and domains with years of good sending history and stable volumes. This suggests an algorithmic trigger rather than a gradual decline.
• IP reputation: 4.7.650 [...] temporarily rate limited due to IP reputation explicitly points to IP reputation as the cause, indicating that Microsoft's internal metrics have flagged the IP for unusual activity or perceived lower quality.
• External monitoring limitations: Public tools like SNDS or Senderscore may still show good (green or high score) reputation, highlighting that Microsoft's internal reputation system can differ significantly from external assessments. This is a common challenge when dealing with Microsoft deliverability issues.
• Transactional emails affected: Even highly critical transactional emails (receipts, notifications, forgotten passwords) from reputable sources like banks and local governments can be affected, indicating a broad filtering adjustment by Microsoft.

Key considerations

• Check for blocklistings: While not always the primary cause for rate limiting, a quick check on major public blocklists is always a good first step. Even if an IP isn't on a public blacklist, private blocklists could be a factor (or internal thresholds). You can check your status on the Microsoft Postmaster Tools (SNDS).
• Assess recent changes: Review any recent, even minor, changes to sending patterns, content, list acquisition, or infrastructure that might have coincided with the bounce increase. Microsoft is sensitive to volume changes.
• Review email content: Even transactional emails can trigger filters if they contain unusual keywords, links, or formatting that mimic spam. Ensure content remains clean and relevant.
• Proactive communication: If all other metrics are strong, directly contacting Microsoft's deliverability support (often via their Postmaster portal or sender support forms) for mitigation is often the most effective next step to resolve the rate limiting.
• Monitor authentication: Ensure your SPF, DKIM, and DMARC records are correctly configured and aligned. Even a slight misconfiguration can impact deliverability, especially with Microsoft's stricter requirements. For instance, sometimes SPF DNS timeouts at Microsoft can cause issues.

Key opinions

• Sudden onset: Many marketers report that issues with Microsoft domains can arise very suddenly, affecting long-standing good senders without prior warning or changes in sending volume.
• Unexplained changes: There's a common sentiment that Microsoft's filtering can be driven by undocumented algorithmic changes or internal 'wonkiness' that isn't immediately transparent.
• Reputation discrepancy: Even with perfect Senderscore ratings and 'green' status on SNDS, marketers still experience bounce issues, suggesting Microsoft uses a more nuanced or internal reputation assessment.
• Mitigation as a last resort: After ruling out common causes like blocklistings or volume changes, marketers often resort to submitting mitigation requests to Microsoft.

Key considerations

• Thorough checking: Always thoroughly check for any blocklistings, even if the error message points to IP reputation. Although direct blocklistings were not found in one case, they should always be considered. Utilize tools for blocklist monitoring regularly.
• Volume stability: While stable volume is good, any perceived spikes or changes in sending patterns can trigger Microsoft's sensitivity, even if they're not a direct increase in total mail volume. Maintain consistent sending habits. More details are available on Mailgun's guide to email bounces.
• Comprehensive metric review: Marketers should review all available email metrics and postmaster/reputation sites, ensuring nothing is missed that might explain the sudden bounce increase.
• Persistence with mitigation: If submitting a mitigation request, be persistent and follow up multiple times to ensure it's addressed.
• Domain integrity: Ensure there is no compromise within the email funnel that could be sending out unintended mail, as this would severely impact your sender reputation and lead to Microsoft bounces (and even an access denied bounce).

Key opinions

• Volume sensitivity: Microsoft is exceptionally sensitive to recent changes in email sending volume, which can quickly trigger rate limiting.
• Funnel compromise: Experts recommend investigating if there has been any compromise in the email sending funnel that could be causing unintended or malicious mail to be sent.
• Algorithmic adjustments: Sudden deliverability shifts could be due to changes in Microsoft's algorithms, filtering criteria, or other internal system 'wonkiness'.
• Mitigation request importance: If other metrics are clean, filing a mitigation request with Microsoft is the best course of action.

Key considerations

• Monitor sending patterns: Even if overall volume hasn't increased, variations in sending patterns (e.g., burst sending, changes in recipient engagement) can trigger Microsoft's filters. Consistent sending is crucial for improving deliverability rates.
• Comprehensive checks: Beyond SNDS, cross-reference all available email metrics and reputation sites to get a complete picture of your sender health.
• Persistence in remediation: When filing for remediation with Microsoft, be prepared to follow up repeatedly, as persistence often pays off.
• Review authentication: Ensure your email authentication, including SPF, DKIM, and DMARC, is fully optimized, as this plays a significant role in Microsoft's trust assessment. You can find more details in our guide to email authentication.

Key findings

• IP reputation focus: Microsoft's bounce messages, like S843, explicitly state that rate limiting is due to IP reputation, indicating their systems closely monitor sender trustworthiness based on IP address.
• Postmaster Tools for insights: Documentation directs senders to Postmaster Tools (like postmaster.live.com) as the primary resource for monitoring their sending reputation, identifying potential issues, and accessing mitigation forms. This suggests these tools provide crucial, though sometimes limited, visibility into Microsoft's filtering decisions.
• Dynamic filtering: While not always explicit, documentation implies that filtering systems are dynamic, constantly adapting to new spam trends and sender behaviors, which can lead to sudden changes in deliverability for senders previously in good standing.
• Authentication importance: Microsoft's sender guidelines frequently emphasize the necessity of proper SPF, DKIM, and DMARC configuration for authentication and improved deliverability.

Key considerations

• Review error codes: Understanding specific bounce codes (e.g., 4.7.650) provided in bounce messages is critical, as they offer direct clues about the nature of the issue. The Microsoft Postmaster provides an email delivery information page.
• Utilize feedback loops: Documentation encourages senders to join programs like Microsoft's Junk Mail Reporting Program (JMRP) to receive feedback on user complaints, which directly impacts IP reputation.
• Adhere to sending guidelines: Microsoft's guidelines for senders (available on their Postmaster site) detail expectations for legitimate email, including best practices for list management, content, and sending infrastructure. Compliance helps avoid reputation issues and subsequent bounces. These guidelines often refer to deliverability issues as a broader category.
• Formal mitigation requests: Documentation provides avenues for submitting delisting or mitigation requests for IPs or domains that have been affected by reputation issues.