New answers tagged exchange
The Yosemite update appears to have fixed the problem. Now running OS X 10.10.1, with Mail version 8.1, the unread marks are updating instantly. UPDATE: I was too optimistic. It's better than it was, but there are still messages marked as unread that I have read.
This is a problem between Microsoft Exchange and your mac. I have had same issue a while ago, and decided not to change stuff via an Apple device. I know it's not the answer you were hoping for, but's the reality at this moment.
As mentioned here as well, you could try to set the following in your Terminal: defaults write com.apple.Mail MarkAsReadDelay 0
Maybe this is because you are connected to that Exchange server. You could try to disconnect from it by taking that account offline (Mailbox -> Online Status -> Take "YourAccountName" offline) and try to edit the account then. If that does not help, try to delete that account and create it again with the corrected credentials.
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