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I might have a problem with my Offline Address Book in Outlook.
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In general, Outlook will automatically update the Offline Address Book (OAB) for Exchange accounts once every 24 hours.For more information about how to manually download the latest copy of the Offline Address Book, click here: You can't find a user in the offline address book in Office 365 If the existing Outlook profile is corrupted, this corruption could be the cause of the issue.Corruption may have occurred if Outlook stopped responding.NOTE: If you change your on-premises Active Directory schema, but the Microsoft Azure Active Directory Sync Tool still doesn't synchronize these changes correctly, go to this Microsoft website to continue troubleshooting this issue: Troubleshoot directory synchronization Manually download the Offline Address Book in Outlook.
This may also resolve nondelivery report issues if you're using an outdated version of the Offline Address Book.
This action is typically done by Outlook in the background.