A client of ours has a mailbox in Office 365 and per his request, we've added that mailbox to his assistant's Outlook to help him weed through his sales emails. Whenever she clicks send/receive and receives new mail, it gets marked as read on the original user's Outlook. Even though the mail hasn't been read by either party. Tried changing reading pane settings, no fix.
If it is configured as an exchange account there is a setting you can change this behaviour in Outlook.
In Outlook 2013 Go to File -> Options --> E-Mail
On the right side there is a menu point "Outlook-Sections". You will find in this section the option "Reading Pane". Click this button an you get a new settings window. In this box un-select the option "Mark item as read when selection changes" (or something like that). As well you may un-select the option "Mark items as read when viewed in Reading Pane".
If you have done this setting the mails will stay unread as far as you only use the quick read area. To set the mails as read you will need to double klick (open) the mail or set the mail as read using the conxext menu.
I hope this will help.