Microsoft is investigating an ongoing Exchange Online outage preventing customers from sending email and triggering 503 errors on affected systems.

Affected users are reporting issues with emails sent from Microsoft 365 environments returning with “503 5.5.1 Bad sequence of commands” errors.

“We are investigating an issue where some users may not be able to send Exchange Online email due to a recent change in availability infrastructure,” Redmond tweeted.

“We are identifying a plan to resolve the impact. Further details can be found under EX649175 in the admin center.”

As the company confirmed in Microsoft 365 admin center ticket EX649175, some users may experience issues when sending Exchange Online emails and see “Bad sequence of commands” errors.

“We are reviewing service monitoring telemetry to isolate the root cause and develop a remediation plan,” Microsoft added.

“The impact is specific to certain users who are served by the affected infrastructure.”

The company has yet to reveal which Microsoft 365 regions are affected by this Exchange Online outage.

However, based on customer reportsthe impact spread to users in the United States, Europe, India, and the United Kingdom.

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

In June, Redmond dealt with another failure preventing US customers from accessing their Exchange Online mailboxes through Outlook on the web.

This is a developing story…