Microsoft is investigating and working to resolve Exchange Online mailbox access issues that have intermittently affected Outlook mobile and macOS users for weeks.
When it first acknowledged this service issue (tracked under EX1256020) last week, Microsoft said it started on March 11 and that the root cause was a newly introduced virtual account. While the company flagged it as resolved on April 1, the incident has been re-added to the admin message center under a different tag (EX1268771).
“We’ve received reports from affected tenants that the impact scenario originally communicated through SHD EX1256020 is still ongoing. We’re working to restart the Notification Broker service on affected portions of Exchange Online service infrastructure to remediate impact while we continue our analysis into the underlying root cause,” Microsoft says.
“Impact may occur intermittently for some users accessing their Exchange Online mailboxes in the Outlook mobile apps or the new Outlook for Mac desktop client. This section may be updated as our investigation progresses.”
In the latest update issued on Thursday evening, the Exchange Online team said it’s still investigating the root cause and working on additional measures to prevent this issue from recurring.
While Microsoft has yet to share which regions or how many users have been affected by this issue over the last three weeks, it flagged this as an incident, which typically applies to critical service issues with noticeable user impact.
Earlier this month, Microsoft mitigated an Exchange Online outage that prevented users from accessing mailboxes and calendars via Outlook on the web, Outlook desktop, Exchange ActiveSync, and other Exchange Online connection protocols.
The same day, it fixed a separate issue that caused Office.com or Microsoft 365 Copilot web sign‑in problems affecting the Microsoft Copilot desktop app, Copilot in Microsoft Teams, and Copilot in Office apps.
Another Exchange Online service outage that intermittently blocked email via the Internet Mailbox Access Protocol 4 (IMAP4) was addressed in January, while a similar incident blocked Exchange Online access via the classic Outlook desktop client in November.
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