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According to my search, I found a thread with similar issue.
To fix it, make sure that the Outlook window is active, and press the ESC key several times. After that, you should be able to use the drag & drop feature again.
Besides, this might also happen if certain Outlook folders are corrupted which causes the problem with drag & drop.
- Close your Outlook.
- Launch the Run command
- Type the following command: outlook.exe /resetfolders and click OK to execute it. This should repair the default Outlook folders and restore the drag & drop capability.
Restart the PC after this and see if this helps you out.
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