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.
Outlook – Office 365 mailbox marking items as read when they aren’t
emailmicrosoft-outlookoffice365
Best Answer
If the assistant is using POP to check for mail, it may mark them as read when it downloads new messages. IMAP and Exchange wait until the message is flagged as "read" by the client or web app.