Microsoft is working to resolve an Exchange Online issue causing email access problems for Outlook mobile users who use Hybrid Modern Authentication (HMA).

HMA is a Microsoft Exchange Server feature that allows users to access on-premises mailboxes using authorization tokens from the cloud.

Tracked under EX1137017 in the Microsoft 365 Admin Center, this known issue is caused by a recent service change, designed to improve the efficiency of mailbox syncs, which is triggering 12-hour sync delays for affected customers.

The issue began impacting Outlook mobile users on August 17, causing some mailboxes to crash and preventing affected customers from accessing their email and calendars.

"Some users may be unable to send or receive email from the Outlook mobile app on their devices," the

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