unmark

UNMARK  -  mark the current message as unseen

Usage:  UNMARK

The UNMARK command changes a message from "seen" to "unseen".  In other
  words, UNMARK marks a 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