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.”