Mark messages as read or unread
In the message list, a blue dot next to a message means you haven’t read it yet. When you select the message to read it, the blue dot disappears. (An unread VIP message uses a blue star instead of a dot.)
Mark one message: Swipe right with two fingers, then click Mark as Read or Mark as Unread.
Mark several messages: Select the messages, then choose Message > Mark > As Read or Message > Mark > As Unread.
Mark all messages in a mailbox as read: In the Mail sidebar, Control-click a mailbox, then choose Mark All Messages As Read. This command is available only if the mailbox contains at least one unread message.