By default, Mailbird will mark each email that you click as read. If you don't want this to happen you can set it up in Mailbird settings
You can change this to never, set a time delay (from 1 to 60 seconds) until Mailbird marks them as read, or never mark messages as read. Remember, if you set the time delay to 5 seconds, that means you need to keep opening that message for 5 seconds so that Mailbird can mark them as read.