at a mailx prompt. (If you omit the letter number, mailx replies to the current letter.) For example, to reply to the sender of letter 2, give the command:
& r 2
mailx automatically addresses your letter and supplies an Re: Subject: line that echoes the original Subject: line. Send your reply like any other letter.
R is a variant of the reply command that sends your reply to all recipients of the original letter as well as to its sender. Use this command only when absolutely necessary, to avoid generating "junk mail."