Summary
Microsoft Office 365 MX deferring connections result from a combination of factors. These include Microsoft-side issues like IP throttling due to anti-spam measures (sometimes triggered by changes in anti-spam rules), broader Microsoft outages rooted in networking problems, and Exchange Online's throttling mechanisms designed to protect service health. External factors, such as network congestion, DNS problems (affecting MX records and SPF/Sender ID checks), recipient server unavailability, and overloaded servers indicated by 451 errors, also contribute. Resolutions involve Microsoft addressing its infrastructure, senders ensuring good IP reputation and email authentication (SPF/DKIM/DMARC), adhering to sending limits, utilizing MXToolbox for DNS diagnosis, implementing proper retry mechanisms, and acknowledging Microsoft's increasing security focus. Monitoring deferred mail offers insights into inbox placement, and following industry best practices is crucial. The system typically retries delivery for 24-72 hours.
Key findings
- Microsoft-Side Throttling: Microsoft's anti-spam procedures unexpectedly limit IP addresses, causing throttling and delayed email delivery.
- Microsoft Outages: Networking issues within Microsoft's infrastructure lead to widespread email deferrals.
- Exchange Online Throttling: Exchange Online uses throttling to protect service health when senders exceed limits.
- DNS Issues: DNS problems, including incorrect MX records and temporary failures in SPF/Sender ID checks, disrupt email delivery.
- Server Overload: 451 errors indicate the receiving server is overloaded or unavailable.
- Security Focus: Microsoft's emphasis on security may cause legitimate emails to be deferred.
- Temporary Problems: Deferred messages often signal temporary problems preventing immediate delivery.
- Inbox Placement: Deferred mail can serve as a signal for monitoring inbox placement.
Key considerations
- IP Reputation Management: Maintain a good IP reputation to avoid being flagged by anti-spam filters.
- Email Authentication: Implement SPF, DKIM, and DMARC to authenticate emails and improve deliverability.
- DNS Health Checks: Regularly check DNS records for errors or outages using tools like MXToolbox.
- Adherence to Sending Limits: Adhere to Exchange Online's sending limits to prevent throttling.
- Retry Mechanisms: Implement robust retry mechanisms to handle temporary server unavailability.
- Best Practices Adherence: Follow email marketing best practices to avoid spam flags and deferrals.
- Service Health Monitoring: Monitor Microsoft service health for known issues and updates.
- Patience and Retry: Understand the system retries delivery automatically and avoid resending immediately.
- Deliverability Monitoring: Implement systems to track deferred connections as a signal for potential deliverability problems.
What email marketers say
5 marketer opinions
Microsoft Office 365 MX deferring connections can stem from various factors including Microsoft outages (often due to networking issues), aggressive anti-spam filtering, DNS problems, and overloaded or temporarily unavailable servers. Resolutions range from Microsoft addressing underlying infrastructure issues to senders ensuring proper email authentication (SPF, DKIM, DMARC), maintaining a good IP reputation, adhering to sending limits, and retrying sending emails later when servers are busy. Microsoft's increasing focus on security may also contribute, so proper configuration and adherence to best practices are crucial.
Key opinions
- Microsoft Outages: Networking issues within Microsoft's infrastructure can lead to widespread email deferrals.
- Aggressive Anti-Spam: Microsoft's anti-spam filtering may inadvertently block legitimate emails, causing deferrals.
- DNS Issues: Incorrect MX records or DNS server outages can prevent proper email delivery and lead to deferred connections.
- Server Overload: The "451 4.7.500 Server busy" error indicates the receiving server is overloaded or temporarily unavailable, requiring retries.
- Security Focus: Microsoft's heightened security measures may increase the likelihood of legitimate emails being deferred.
Key considerations
- IP Reputation: Maintain a good IP reputation to avoid being flagged as a spam source.
- Email Authentication: Implement proper SPF, DKIM, and DMARC records to authenticate your emails and improve deliverability.
- DNS Health: Monitor your DNS records for errors or outages that could impact email delivery.
- Sending Limits: Adhere to Microsoft's sending limits to avoid throttling and deferrals.
- Retry Mechanism: Implement a robust retry mechanism to handle temporary server unavailability.
- Best Practices: Follow email marketing best practices to ensure your emails are not flagged as spam.
Marketer view
Email marketer from Reddit shares that Microsoft's increasing focus on security and spam filtering can lead to legitimate emails being deferred. They advise ensuring your sending infrastructure is properly configured and that you are following email best practices.
18 Sep 2023 - Reddit
Marketer view
Email marketer from Reddit shares that they experienced similar issues due to Microsoft's anti-spam filtering aggressively blocking IPs. The user suggests checking your IP reputation and ensuring proper SPF/DKIM/DMARC records are in place as a proactive measure.
7 Nov 2023 - Reddit
What the experts say
6 expert opinions
Microsoft Office 365 MX deferring connections can arise from IP address throttling by Microsoft's anti-spam measures, often due to changes in anti-spam rules. Resolution initially involves manual whitelisting of IPs, followed by attempts at developing and deploying fixes, potentially involving code reversions. Temporary DNS problems impacting SPF/Sender ID checks also contribute to temporary failures. Monitoring deferred mail can serve as an inbox placement signal, and proper handling of deferred emails necessitates retrying delivery attempts within 24-72 hours, aligning with industry best practices for legitimate email infrastructure.
Key opinions
- IP Throttling: Microsoft's anti-spam procedures can unexpectedly limit IP addresses, causing throttling and delays in email delivery.
- Anti-Spam Rule Changes: Recent changes to Microsoft's anti-spam rules can lead to IP address limitations.
- Fix Development Challenges: Developing and deploying fixes for throttling issues can be challenging, potentially requiring code reversions.
- SPF/Sender ID Failures: Temporary DNS problems can cause SPF and Sender ID checks to fail, leading to temporary failures and deferred connections.
- Inbox Placement Signal: Deferred mail can be used as a signal for monitoring inbox placement and identifying deliverability issues.
- Retry Importance: Proper handling of deferred emails includes retrying delivery attempts within 24-72 hours.
Key considerations
- IP Whitelisting: Report affected IP addresses for manual whitelisting to provide immediate relief from throttling.
- DNS Monitoring: Monitor DNS for temporary problems impacting SPF and Sender ID checks.
- Deliverability Monitoring: Set up systems to track deferred connections and identify issues affecting deliverability.
- Retry Implementation: Ensure your email infrastructure properly queues and retries deferred messages.
- Industry Best Practices: Align your email infrastructure and practices with industry best practices for handling deferred emails.
Expert view
Expert from Word to the Wise explains the importance of properly handling deferred emails by retrying delivery attempts. They emphasize that legitimate email infrastructure should be capable of queueing and retrying deferred messages according to industry best practices. Sending servers will keep retrying for 24-72 hours.
18 Jan 2023 - Word to the Wise
Expert view
Expert from Word to the Wise shares that deferred mail can be used as a signal for inbox placement monitoring. By setting up systems to track deferred connections and mail, senders can identify issues that may be affecting deliverability to certain domains.
22 Dec 2023 - Word to the Wise
What the documentation says
3 technical articles
Microsoft Office 365 MX deferring connections, indicated by deferred messages and 451 errors, are caused by temporary problems such as network congestion, DNS issues, unavailable recipient servers, and Exchange Online throttling. Throttling is a mechanism to protect service health and stability when senders exceed defined limits. Resolution involves checking service health, adhering to sending limits and best practices, and understanding that the system will automatically retry delivery for 24 hours.
Key findings
- Temporary Problems: Deferred messages and 451 errors signal temporary issues preventing immediate delivery.
- Network Congestion: Network congestion can contribute to email delivery delays.
- DNS Issues: DNS problems are a potential cause of deferred connections.
- Server Unavailability: Recipient server unavailability leads to deferral.
- Exchange Online Throttling: Exchange Online uses throttling to protect service health.
- Automatic Retries: The system will automatically retry delivery for 24 hours after a deferral.
Key considerations
- Service Health Check: Check Microsoft service health dashboards for known issues.
- Adhere to Limits: Adhere to Exchange Online's sending limits to avoid throttling.
- Best Practices: Follow email sending best practices to prevent deferrals.
- Patience: Understand that the system will automatically retry delivery, so avoid resending immediately.
Technical article
Documentation from Microsoft Learn explains that Exchange Online employs throttling mechanisms to protect the service's health and stability. Throttling can cause connection deferrals and delays if a sender exceeds defined limits. Resolution involves adhering to Microsoft's sending limits and best practices.
16 Sep 2022 - Microsoft Learn
Technical article
Documentation from Office365 explains that a 451 error usually suggests a temporary issue so the service will keep attempting delivery for 24 hours.
2 May 2022 - Office365
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