Mac Mail shows unread messages which I can not find

It really funny. I set up several mailboxes in mail on my Mac, one is google which works fine, another two are from my university which use Microsoft exchange server. At the beginning, everything works perfect. However, recently one mailbox using Microsoft exchange shows there are 1 unread message, and so does the inbox of my mail. But I cannot find any unread message in that mailbox, even on the server where I searched really carefully.

I searched on google, but I still have not solved this issue. There is a way to "solve" this issue to some extend.  At least, the mail on the dock shows the right number of the unread messages.

Mail -> Mailbox -> New Smart Mailbox, when you create a new smart mailbox, you are prompt to input the name of smart mailbox. I name it as "Unread messages"(Of course you can name whatever you want). And then choose the rule condition as "Message is unread". Now on the left panel of your mail there is a SMART MAILBOXES menu.

Then, Mail -> Preferences -> General -> Dock unread count, choose the smart mailbox you just created. Now the number of unread message on the dock is correct.

I am still keep my eyes on this issue, hopefully I could find a way to solve this issue soon.