1. Check the settings for your email client (eg. Outlook) to see if there are any auto-mark read settings
2. Reduce the number of visible emails when refreshing/opening your mailbox; this will load faster and prevents emails from being marked as read
3. Try setting a keyboard shortcut to mark emails as unread or use the mouse to click on the specific emails you want to mark as unread.
4. Check to see if any kind of triggers/filters have been enabled which could possibly mark emails as read automatically.