There is no option available in
Exchange server side to manage mailbox read or unread status.
But In Outlook, we can set
the following settings: In Outlook 2010/2013, click View > Reading Pane >
Options. The default is set to mark items as read when selection changes.
If you disable this option,
all mails remain as unread unless users manually mark it as read or double
click the message to open it in its own window.