Microsoft says it partially mitigated a week-long Exchange Online outage causing delays or failures when sending or receiving email messages.

While the company didn’t publicly share information on this incident, it tagged it as a critical service issue tracked under EX1027675 on the Microsoft 365 Admin Center.

Microsoft has yet to share more information on what regions were affected by this outage, but it said the incident impacted “any user serviced by the impacted portion of infrastructure.”

Customers worldwide also reported experiencing email delivery failures over the last week, with those impacted saying they were receiving a Non-Delivery Report (NDR) with a “554 5.6.0 Corrupt message content” error.

The company first acknowledged the Exchange Online email delivery issues on March 10, 11:14 AM, but the admin center incident report says the outage started on March 7, 12:30 PM UTC.

“A recent service update, intended to improve our message transport services, introduced a code issue that resulted in impact for a portion of service infrastructure,” Redmond said in the final update regarding this incident on Thursday.

“Additionally, users may be unable to send email messages with attached files in any connection method of Exchange Online. Sending attachments as ZIP files allows the email messages to be delivered as expected, serving as a method by which to bypass the issue while we continue to investigate.”

Similar issue still causing email delivery failures

Microsoft continues to track a separate incident with a nearly identical impact under EX1030895 after successfully mitigating EX1027675 with a fix that rolled out Wednesday morning.

This second incident is still ongoing, with the company saying it’s “limited to a small subset of messages” that trigger NDR failures and intermittent plain text calendar invite emails with winmail.dat attachments.

Although it is still working on finding the root cause of these email delivery failures, Redmond says it’s testing a potential fix deployed on an isolated section of its infrastructure.

“We’re conducting multiple targeted restarts of machines containing the fix to ensure the expected relief is achieved,” Microsoft said earlier today. “Additionally, we’re closely monitoring our diagnostic telemetry data to isolate the root cause of the issue, and we’re reviewing the overall service health during this process.”

Two weeks ago, the company linked a weekend Microsoft 365 outage affecting Outlook and Exchange Online authentication to another “code issue.” A subsequent advisory revealed that Exchange Online users still had issues accessing calendars and email messages using the iOS native mail app.

Days later, it mitigated one more Microsoft 365 outage affecting Teams customers and causing call failures.