Using Yosemite (OSX 10.10) and the included Mail application (which describes itself as "Version 8.0 (1990.1)" to read mail held on a company Exchange server, I am finding that the unread marks for messages are not being reliably updated. Before the Yosemite upgrade, whenever I selected the message summary in the left hand pane, it was instantly marked as "read". This no longer happens reliably: some messages are marked as read, others are not. For example, in this screenshot, I have clicked on all of these messages, and some have updated and some have not:
I have also noticed that the unread count on the Inbox does not behave consistently: sometimes it is decremented even though the message still shows as Unread, sometimes the number does not change.
In case it matters, I'm running on an Early 2011 MBP with 8Gb RAM.
This seems to be only an issue with the Exchange account. I have two other mail accounts in Mail, and have no issues with either of them. I've looked at the settings for my Exchange account, and can't see anything that seems unusual.
Has anyone else experienced this and resolved the problem?