unmark

UNMARK  -  mark the current message as unseen

Usage:  UNMARK

Please note that UNMARK acts on the "current message" in your mail file,
  and not on the outgoing message.  If you are REPLYing, the current
  message is the one you are replying to.

The UNMARK command changes the current message from "seen" to "unseen".  
  In other words, UNMARK marks the message as "unseen", that is, it puts a
  U indicator on the header line.  (For messages that arrived during the 
  current session, UNMARK will change the indicator from R [recent] to N 
  [new, that is, R + U].)  

Since the READ command, with no message sequence, means READ UNSEEN, using 
  the UNMARK command will make a message appear again the next time you
  type READ to see unseen messages.  This is a way to remember to read
  something again later, for example if you could not finish reading a 
  long message.

For more help type "help" and one of these topics:
  mark   message-sequence   headers   message-tagging