The MARK command sets a flag in the message header that
    identifies the current message or message-identification number
    as marked. Marked messages have an asterisk (*) in the left
    column of the directory listing. A marked message can serve as
    a reminder of important information. The /ALL qualifier sets all
    currently selected messages as marked.

    The UNMARK command clears the flag in the message header and
    unmarks the current message or message-id number; that is, the
    asterisk is deleted.

    Formats

      MARK  [/ALL] [message-number]

      UNMARK  [/ALL] [message-number]