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Hello @Sara T
Could you provide more details, what version of outlook are you currently using? Have any changes been made recently?
Have you tried syncing manually or viewing the sync logs? If the OST file is damaged, you can try to repair or recreate it.
Please check the method in that link, hope this helps you.
(Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)
Also, you can check out this.
Synchronization problems occur in Outlook and in Outlook Web App
Regards
SF
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