The REPLY command is synonymous with the ANSWER command. It sends
    a message to the sender of the message you are currently reading
    or to the sender of the message read last. If you do not specify
    the name of a file to be sent as your reply, Mail prompts you for
    the text of your reply.

    Note that you must be reading a message in order to reply to it.

    If you change your mind about replying to a message after
    entering the REPLY command, press Ctrl/C to abort the message
    and return to the MAIL> prompt.

    Format

      REPLY  [file-spec]