Microsoft Exchange Online service change causes email access issues

Microsoft is working to address an ongoing service issue that has intermittently prevented some users from accessing their cloud-based Exchange Online mailboxes via Outlook mobile and Mac desktop clients since Thursday.

After investigating the incident (tracked under EX1256020), Microsoft found that the root cause was a newly introduced virtual account.

On Saturday, began working to revert the change as a possible long-term solution to mitigate the impact, after failing to address the problem by restarting the affected infrastructure.

“This issue may intermittently impact some users who are attempting to access the Exchange Online service through the Outlook mobile apps or the new Outlook for Mac desktop client,” Microsoft said.

“After further assessment, we’ve confirmed that the identified change within the Exchange Online service intended to introduce a new virtual account resulted in impact. To remediate this issue, we’re disabling the change across the affected environments. Once available, we’ll provide a resolution timeline.”

While Microsoft didn’t share which regions or how many users were affected, it flagged this service outage as an incident, which usually applies to critical service issues with noticeable user impact.

One week ago, Microsoft addressed another Exchange Online outage that was preventing customers from accessing their mailboxes and calendars via Outlook on the web, Outlook desktop, Exchange ActiveSync, and other Exchange Online connection protocols.

On the same day, it fixed a separate issue causing Office.com or Microsoft 365 Copilot web sign‑in access problems due to what the company described at the time as “a high volume of traffic,” affecting the Microsoft Copilot desktop app, Copilot in Microsoft Teams, and Copilot in Office apps.

Microsoft also mitigated an Exchange Online service outage in January that intermittently blocked email via the Internet Mailbox Access Protocol 4 (IMAP4) and a similar incident in November that blocked Exchange Online access via the classic Outlook desktop client.

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