Apple Mail shows read messages as unread.

[tfindlay]

First it was one, then three, and now six. Apple Mail is showing 6 unread messages but when I click on the Unread mailbox it is empty. I have Googled this problem and it seems to be quite common and yet no one seems to know how to fix it. Any ideas?

 

[chas_m]

The unread messages are in your second account. Click the little carat next to "all inboxes" to reveal your other account.

 

[tfindlay]

The two accounts are iCloud and Google. I checked my Gmail account and there are no unread messages there.