unanswer

UNANSWER  -  remove the A indicator from the current message

Usage:  UNANSWER

Please note that UNANSWER acts on the "current message" in your mail file,
  and not on the outgoing message.  

(This command is useless if you are REPLYing, since the current message,
  the one you are replying to, will be marked ANSWERED as soon as you send
  the reply.)

The UNANSWER command removes the A indicator from the header line of
  the message.  The A shows that you sent a REPLY or ANSWER.

For more help type "help" and one of these topics:
  headers   message-tagging   top-level-mode reply