Hidden unread message in inbox

I am using apple mail on my Mac desktop (OS X Yosemite version 10.10.5).

It is showing I have 1 unread message in my inbox but I can't see it. The same thing is happening on my MacBook but not my iPhone or iPad.

I have tried multiple things like sorting them by 'unread'; marking all messages as unread and then all as read; emptying the trash, etc. etc. I have even deleted my account and re-set it up. I have expanded all conversations and looked through all 6000 messages.

My suspicion is that it has something to do with a calendar event invitation maybe?

Any suggestions on how to fix this?!