flag

FLAG  -  mark the current message so it stands out

Usage:  FLAG 

The FLAG command puts a "flag" or marker on the message.  The letter "F"
  appears next to it in the header line.  By default, when you start
  up MM you see headers of flagged messages as well as new ones.  You
  can also use "flagged" as a message-sequence.

Examples:
    Read>flag 		#flags the message
    MM>read flagged	#reads flagged messages

The UNFLAG command will remove the flag.

For more help type "help" and one of these topics:
  message-tagging   unflag   headers   set display-flagged-messages