It can be annoying when you’ve finished reading your emails, and yet Outlook’s inbox is flagged with one or more unread messages. Even when that’s clearly not the case. It can be more than one cause, so here are some tips:
The first method works inside Outlook:
The second method involves going to the web access:
Use the third method if you’re on an Exchange server, Office 365 or Outlook:
This link from Microsoft explains how to turn cached exchange mode on and off: Turn on Cached Exchange Mode – Outlook (microsoft.com)
The last method is the nuclear option, and it will also reset any custom views you’ve created.