Mark emails as read or unread in Mail on Mac
Email messages you haven’t read yet have a blue dot or star (for messages from VIPs) next to them. When you select a message to read it, the dot or star disappears. If you want, you can manually mark messages as read or unread.
In the Mail app on your Mac, select one or more read or unread messages.
Choose Message > Mark > As Unread or Message > Mark > As Read to change the status.
To mark one message, you can also swipe right with two fingers on the trackpad, then click Mark as Unread or Mark as Read.