Sets a flag in the message header setting the current or message-id message
  as marked. Marked messages are displayed with an asterisk in the left
  hand column of the directory listing. A marked message can serve as a
  reminder of important information. The UNMARK command clears a flag in
  the message header setting the current or message-id message as unmarked.

  The /ALL qualifier sets all currently selected messages as
  marked or as unmarked.

   Format:

       MARK [/ALL] [message-number]
       UNMARK [/ALL] [message-number]

Additional information available:

ParametersQualifiersExamples

 message-number

  Indicates message number to be marked or unmarked.

Additional information available:

/ALL

 /ALL

 Sets all currently selected messages as marked. Marked messages are
  displayed with an asterisk in the left-hand column of the directory
  listing.

  1.
       MAIL> DIR MISC

            #          From          Date           Subject

            1         SILVER:SMITH   13-AUG-1986    Training Info
            2         GOLD::COLLINS  22-AUG-1986    Ideas
            3         GOLD::PETERS   25-AUG-1986    Meeting

       MAIL> MARK 2
       MAIL> MARK 3
       MAIL> DIR

            #          From          Date           Subject

            1         SILVER:SMITH   13-AUG-1986    Training Info
           *2         GOLD::COLLINS  22-AUG-1986    Ideas
           *3         GOLD::PETERS   25-AUG-1986    Meeting

  This example shows how to MARK messages 2 and 3. Use the UNMARK
  command to delete the asterisk and unmark the message.