mark

MARK  -  mark the current message as seen without reading it

Usage:  mark

Please note that MARK 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 MARK command marks an UNSEEN message as SEEN.  It is very rarely
  used in SEND MODE, since normally the current message really was
  SEEN.  

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