If after reading an email, Outlook does not automatically mark it as read or marks it as read after first selecting another email, continue reading below to fix the problem.
Problem in details: When you read an email using the Reading Pane in Outlook, the email is not marked as read immediately, or it is marked as read after you select another email. (The issue can appear in Outlook 2019, 2016, 2013 OR 2010).
FIX: Outlook emails doesn’t marked automatically as read after reading them in Reading Pane.
Method 1. Modify Reading Pane options.
By default, Outlook marks an email message as read after you open it, or select it and read it using the reading pane. But, when using the reading pane, Outlook sets an email as read after you select another email item. To set Outlook to mark the emails as read immediately, do the following:
1. In Outlook View tab, click Reading Pane > Options.
2. In Reading Pane options, apply the following settings and click OK:
- Check the option Mark items as read when viewed in the Reading Pane
- Set the delay to 0 seconds before marking item as read.
- Uncheck the option Mark item as read when selection changes.
3. Now select/read a message and normally it will be marked as read immediately. If not, continue to the next method.
Method 2. Disable Third-Party Add-ins.
If Outlook doesn’t mark immediately the emails as read (after you read them), then probably a third-party Add-in causes the issue.
1. Press Windows 
+ R keys to open the run command box.
2. Type: outlook /safe and click OK to open Outlook in Safe Mode (without the Add-ins loaded).
3. Read an unread message in reading pane and check if it is marked as read or not. Then, according the result, proceed as follows:
- If the issue persists, proceed to next method.
- If the issue is solved, proceed to find out which add-in cause the problem, by doing the following:
a. Click the File menu and select Options.
b. Select Add-ins at the left pane and at the right pane, select COM Add-ins and click Go…
d. Uncheck all the checkboxes to disable all the Add-ins and click OK
e. Restart Outlook and re-enable one-by-one all the disabled Add-ins and restart Outlook, until you find which one causes the problem.
Method 3. Mark All Messages as READ and then as UNREAD again.
This method is a little tricky, but sometimes solves the mentioned issue.
1. Press CTRL + A to select all messages and then right-click and select Mark As Unread.
2. Perform the same steps and mark again all the messages as Read.
3. Finally, mark 2-3 messages as Unread to check if the issue persists.
Method 4. Repair Office installation.
1. Open Programs and Features in Control Panel.
2. In Programs and Features, select the Microsoft Office product that installed on your system and click Change.
3. Select Repair and click Continue.
4. Select the Quick Repair option and then wait the Office repair to complete.
5. After repair, open Outlook and see the problem is fixed. If not, follow the same steps but this time choose the Online Repair option to reinstall your office product (requires an Internet connection).
That’s all folks! Did it work for you?
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