Right-click Inbox - Mark Folder Read

I thought this solved it but it doesn't. Doing this means the whole of the folder appears read. But I just want the unopened message to be opened.

If you don't see an unread message, then there is none. The unread notification you see is a display error.

Actually I've discovered how to display unread messages. There's an icon next to the filter bar at the top, which does just that. But thanks anyway.