If you omit the message number, mailx replies to the current message.
For example, to reply to the sender of message 2, type the following command.
& r 2
mailx automatically addresses your message and supplies an Re: Subject: line that echoes the original Subject: line. Send your reply as you would with any other message.
R is a variant of the reply command that sends your reply to all recipients of the original message as well as to its sender. Use this command only when absolutely necessary, to avoid generating “junk mail.”