Exchange Online outage hits Microsoft 365 users worldwide

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As reported by BleepingComputer, Microsoft is investigating an ongoing Exchange Online outage preventing customers from sending emails and triggering 503 errors on affected systems. Impacted users report having issues with emails sent from Microsoft 365 environments being returned with “503 5.5.1 Bad sequence of commands” errors.

“We’re investigating an issue where some users may be unable to send Exchange Online email messages due to a recent change to the free/busy infrastructure,” Redmond tweeted. “We’re identifying a plan to resolve the impact. Further details can be found under EX649175 in the admin center.”

As the company confirmed in the EX649175 ticket in the Microsoft 365 admin center, some users may experience issues when sending Exchange Online emails and see “Bad sequence of commands” errors. “We’re reviewing service monitoring telemetry to isolate the root cause and develop a remediation plan,” Microsoft added. “Impact is specific to some users served through the affected infrastructure.”

The company has not revealed which Microsoft 365 regions this Exchange Online outage affects. However, based on customer reports, the impact has spread to users in the United States, Europe, India, and the United Kingdom.

Two weeks ago, Redmond also addressed an issue preventing users from searching their emails and triggering 401 exception errors saying, “Sorry, something went wrong. Please try again later.”

As of July 18 at 06:39 EDT, Microsoft has removed the EX649175 incident from the admin center and says the outage’s root cause has been addressed. However, customers still report being affected and experiencing issues when sending Exchange Online emails.