How to Steps
Mark a message as read or unread on Outlook 2016
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1. Right-click the message.
2. Click Mark as Read or Mark as Unread.
3. To mark several messages as either read or unread at once, click the first message, hold down Ctrl, click the other messages, and then select the option you want.
4. Repeat to toggle a message status from read to unread and back again.