Why did Microsoft 365 block Google’s and LinkedIn’s domain? What is MO255463?

“Your most unhappy customers are your greatest source of learning.” – Bill Gates

Microsoft issued an advisory (MO255463) notifying its users and admins that a Microsoft 365 bug has resulted in a massive flow of legitimate emails sent from multiple domains (including Google and LinkedIn) getting tagged as malicious and quarantined.

Impact Of The Microsoft 365 Bug

  • inability to send/receive email from various domains (incl. Google & LinkedIn)
  • quarantining legitimate emails
  • inability to access links within emails as they’ve been flagged as ‘high risk’
  • users potentially getting blocked for sending emails if messages are incorrectly classified as spam or phishing

According to Microsoft, this whole unfortunate fiasco has been associated with – 

  • an increase in the number of URL-related alerts for non-malicious URLs
  • an increase in the number of Zapped Phish AIR investigations in Microsoft Defender
  • legitimate emails being marked as malicious in Threat Explorer
  • delays in getting the latest email information in Threat Explorer

This year definitely didn’t get off to a good start for Microsoft. First, it was the Microsoft Exchange hack dubbed “The Greatest Email Heist” back in March. Then the FBI got involved with a court-authorised fix for the hack. Not to mention Egress’ research stating that 85% of organisations using Microsoft 365 have had an email breach in the last 12 months.

Microsoft reported all issues – including the current one – have been fixed, but… how long until another hack or “issue” comes along threatening the email data security of millions of organisations and individual users?

Final Thoughts

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