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

.ip \fBreply\fP
Frame a reply to a single message.
The reply will be sent to the
person who sent you the message to which you are replying, plus all
the people who received the original message, except you.  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.