Microsoft has resolved an issue that caused a spam rule to flag all outgoing emails as junk for some users of its Exchange Online Protection (EOP) service.

EOP is a cloud-based email filtering service that protects Exchange Online users from spam, malware, phishing, and other email threats. It also provides outbound email scanning to prevent users from sending malicious or unwanted messages.

On October 17, 2023, Microsoft acknowledged that some EOP customers were experiencing problems with a spam rule that incorrectly marked all sent emails as junk. The issue affected both Outlook and Outlook on the web users who had the junk email filter enabled.

According to Microsoft, the faulty spam rule was deployed as part of a regular update to improve the accuracy and effectiveness of EOP. However, the rule had a bug that caused it to misclassify legitimate emails as spam.

Microsoft said that it disabled the spam rule within hours of detecting the issue and restored normal email functionality for affected users. The company also apologized for the inconvenience and assured customers that it was taking steps to prevent similar incidents in the future.

How to prevent your emails from being marked as junk

While Microsoft has fixed the issue with the faulty spam rule, there are some general best practices that you can follow to reduce the chances of your emails being marked as junk by EOP or other email filtering services. Some of these best practices are: