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For better research, please provide more details about this case:
Does the problem occur in the Outlook desktop client?
What type did you add to Outlook(POP/IMAP/Exchange)?
Here are a few steps you can try to resolve this:
Check Sync Settings: Ensure that your Outlook app is set to sync all emails.
Update Outlook: Make sure both your Outlook app and web version are up to date. Sometimes, updates can fix sync issues.
Check Filters and Rules: Ensure that there are no filters or rules set up in your Outlook that might be causing emails to be hidden or moved to different folders.
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