What causes Microsoft Office365 MX deferring connections and how is it being resolved?

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.

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.

Email marketer from StackOverflow responds that the "451 4.7.500 Server busy" error often indicates that the receiving mail server is overloaded or temporarily unavailable. The solution is generally to retry sending the email later, as the server should eventually recover.

Email marketer from MXToolbox explains that deferred connections can be caused by DNS issues. They recommend using MXToolbox's tools to diagnose DNS problems, such as incorrect MX records or DNS server outages, that could be preventing proper email delivery.

Email marketer from Thurrott.com details an instance where a Microsoft outage affected Office 365, causing email delays and deferrals. The cause was a networking issue that impacted multiple services. The resolution involved Microsoft engineers identifying and resolving the network bottleneck.

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.

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.

Expert from Spam Resource explains that one cause for temporary failures (and thus deferred connections) is when recipient servers are doing SPF and Sender ID checks that fail due to temporary DNS problems.

Expert from Email Geeks shares an update that the fix developed by Microsoft to resolve the throttling issue has proven unsuccessful. Microsoft is developing a new fix that will undergo internal validation before deployment.

Expert from Email Geeks shares an update stating that specific IP addresses are being unexpectedly limited by Microsoft's anti-spam procedures, causing inbound external email delivery to become throttled and delayed. Microsoft is reviewing recent changes to anti-spam rules and manually adding reported affected IP addresses to an allowed list for immediate relief.

Expert from Email Geeks provides the final status update: Microsoft has completed the deployment of a change, which was a targeted reversion of the code issue causing impact. Users should now be experiencing mitigation, though some residual delays may still occur over the next hour as the change completes.

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.

Documentation from Office365 explains that a 451 error usually suggests a temporary issue so the service will keep attempting delivery for 24 hours.

Documentation from Microsoft Support explains that deferred messages indicate a temporary problem preventing immediate delivery. These delays may stem from network congestion, DNS issues, or the recipient server being temporarily unavailable. Microsoft recommends checking service health and retrying later.