pgaffey Said:
"I seem to receive all my messages in Mail, but suddenly they do not appear in the right hand window. I get the message 'No message selected,' even when I have selected a message. If I try to open the message, an empty window opens. How to fix?"
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Troubleshooting the Mail App GUI:
Reinstall the macOS:
Nothing would be deleted. It's just that items that have become corrupted or deleted, would be replaced or reinstalled back to where they should be. Go Here: How to reinstall macOS from macOS Recovery - Apple Support. Important: (applies if you can log in) Be certain to create a backup of your Mac prior to all of this. That way, you can have something to restore from the Mac from, should anything go wrong with the reinstall. Once reinstalled, try this once more, seeing if you still get this error.