If Outlook shows read emails as unread, there might be a synchronization issue. It could also be caused by a settings misconfiguration or a minor bug. Here are several ways to fix this problem:

1. Check Reading Pane Settings

• Go to File > Options > Mail > Reading Pane.
• Ensure the “Mark items as read when viewed in the Reading Pane” option is checked.
• Adjust the delay time (e.g., 2 seconds) to ensure the email has enough time to be marked as read.
• Click OK to save changes.

2. Manually Mark Emails as Read

If automatic marking isn’t working:

• Select the unread emails.
• Press Ctrl + Q to mark them as read (on Windows).
• Alternatively, right-click the email and select Mark as Read.

3. Update Outlook

• Outdated versions of Outlook might cause such issues.
• Go to File > Office Account > Update Options > Update Now.

4. Repair the Outlook Data File

• Go to File > Account Settings > Account Settings.
• Select the email account and click Repair.
• Follow the on-screen instructions to repair the account.
• Alternatively, use the Inbox Repair Tool (scanpst.exe) to repair your Outlook data file.

5. Check Synchronization Issues

If you’re using an Exchange, IMAP, or Office 365 account:

• Go to File > Account Settings > Account Settings > Double-click your email account.
• Ensure “Use Cached Exchange Mode” is enabled for Exchange accounts.
• If syncing doesn’t resolve, remove and re-add the account:
  • File > Account Settings > Remove Account.
  • Restart Outlook, then re-add the account.

6. Clear Cached Outlook Files

• Close Outlook.
• Navigate to C:\Users\YourUsername\AppData\Local\Microsoft\Outlook\ and delete cached files.
• Reopen Outlook to recreate the cache.

7. Disable Add-Ins

Faulty add-ins can interfere with Outlook:

• Go to File > Options > Add-ins.
• Select COM Add-ins and click Go.
• Uncheck unnecessary add-ins and restart Outlook.

8. Reset View Settings

• Go to the email folder where the issue occurs.
• Click View > Reset View.

9. Check for Rules or Filters

• Go to File > Manage Rules & Alerts.
• Ensure there’s no rule affecting the read/unread status of emails.

10. Test in Safe Mode

• Open Outlook in Safe Mode to check if the issue persists:
  • Press Windows + R, type outlook.exe /safe, and press Enter.
  • If the issue doesn’t occur, it’s likely due to add-ins or custom settings.

For more: Microsoft Support Forum

If none of these solutions work, reinstalling Outlook or contacting Microsoft Support may be necessary. Let me know if you need detailed steps for any of these fixes! (Contact US)

Detailed video below: