I recently received a new MacBook Pro and restored all of my previous applications and data from a Time Travel backup. One small issue that I noticed afterwards was that the Unread mail count (1) was incorrect, since even when I set a Filter to show only unread items there were no remaining mails shown. Despite a quick search for the answer online it seems that Office 365 (Outlook 15.0) for Mac is not widely written about yet. The solution I fell upon was quite simple (please be careful to check that your mailbox is correctly synchronised before beginning):
- Select the folder which shows the incorrect item count.
- Choose Properties on the folder.
- Click Empty Cache, in order to remove the local copies of the mailbox folder items (this assumes you’re using the Exchange mailbox as a primary store and not a POP server etc)
All mail items were then immediately removed from the local mailbox cache, following which you can right click on the folder concerned and then choose Synchronise Now.
This simple fix easily resolved my problem.