Abstract:
Microsoft recently confirmed that a spam filtering rule previously updated by its Outlook email service had an error, resulting in a large number of legitimate emails being misjudged by the system and directly rejected. Affected users may have missed important emails in the past few days, and these rejected emails cannot be retrieved because they failed to successfully reach the server.
The incident first attracted the attention of some users in early November. Some users found that their Outlook.com and MSN.com mailboxes suddenly became extremely quiet, and they even failed to receive verification emails when registering or verifying third-party platform accounts. This inability to receive emails was initially subtle as the account itself was still functioning normally.
As more and more users report similar problems, investigation results show that the problem is not caused by individual account settings, but related to the Outlook mail system. During the testing process, a large number of emails began to be returned around November 5, and some users reported that the relevant situation occurred even earlier.
Further inspection revealed that the problem mainly affects emails sent to Microsoft email domain names such as Outlook.com, Hotmail.com, Live.com, and MSN.com. Some senders received bounce messages indicating that the email was rejected due to a lack of authentication records such as SPF, DKIM or DMARC.
However, the reality is not that simple. Even if some senders have configured complete and effective SPF, DKIM and DMARC verification mechanisms, the emails they send are still blocked by the system. This indicates that the problem is not with the sender's configuration, but with an abnormality in Microsoft's own email filtering rules.
Regarding this failure, Microsoft stated that the problem of some emails failing to be successfully delivered to Outlook.com, Hotmail and Live email addresses has been resolved. Currently, the email delivery function has returned to normal, and the company is continuously monitoring related services to ensure that emails can be delivered as expected.
Microsoft further explained that the incident stemmed from a recent update to the company's spam rules. Because this rule misidentified some legitimate emails, the system directly rejected them. After receiving a large amount of user feedback, Microsoft discovered the problem and has withdrawn the relevant update.
Microsoft emphasizes that users do not need to modify their personal mailbox settings, and senders do not need to additionally adjust authentication records such as DKIM, SPF or DMARC, because these configurations are not the cause of the problem.
According to information provided by Microsoft, mail traffic returned to normal at 12:02 CET on November 9. If individual users still encounter email delivery problems, they can submit email tracking records to Microsoft for further technical support.
However, Microsoft stated that it cannot restore emails that have been rejected by the system. Because these messages are intercepted in transit and never enter the inbox, spam folder, or any other storage location, there are no copies on the server or local device.
This means that even using Outlook's search function, these lost emails cannot be found. For affected users, the only feasible way is to contact the relevant sender to resend the email, otherwise some missed important emails may be permanently lost.
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