I've looked at the settings for my Exchange account, and can't see anything that seems unusual. I have two other mail accounts in Mail, and have no issues with either of them. This seems to be only an issue with the Exchange account. In case it matters, I'm running on an Early 2011 MBP with 8Gb RAM. 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. Users can connect supported versions of Outlook to Exchange Online, so they can use the rich client application they already know For web-client access, Outlook on the web provides a premium browser-based experience that matches the look and feel of the full Outlook client The Focused Inbox makes it easy to track email that matters most. For example, in this screenshot, I have clicked on all of these messages, and some have updated and some have not: This no longer happens reliably: some messages are marked as read, others are not. Before the Yosemite upgrade, whenever I selected the message summary in the left hand pane, it was instantly marked as 'read'. 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. Mac PowerTalk - 16 Files: Direct Dialup Mail 1.1.2 Extension to PowerTalk: 1995: 1.26 MB: MailLink for PowerTalk/INET 1.6 Personal Gateway for PowerTalk Users: 1995: 1.46 MB: MailLink for PowerTalk/MS 1.6 Personal E-Mail Gateway for PowerTalk Users: 1995: 1.04 MB: MailLink for PowerTalk/QM 1.6 Personal Gateway for PowerTalk Users: 1995: 1.2.

Mac mail exchange gui