V8/usr/src/cmd/Mail/manual/cmds/Reply

.ip \fBReply\fP
Note the capital R in the name.
Frame a reply to a one or more messages.
The reply (or replies if you are using this on multiple messages)
will be sent ONLY to the person who sent you the message
(respectively, the set of people who sent the messages you are
replying to).
You can
add people using the
.b ~t
and
.b ~c
tilde escapes.  The subject in your reply is formed by prefacing the
subject in the original message with
.q "Re:"
unless it already began thus.
If the original message included a
.q "reply-to"
header field, the reply will go
.i only
to the recipient named by
.q "reply-to."
You type in your message using the same conventions available to you
through the
.b mail
command.
The
.b Reply
command is especially useful for replying to messages that were sent
to enormous distribution groups when you really just want to
send a message to the originator.  Use it often.