At the question mark (?) prompt, you can use the R (reply to sender) subcommand to reply to a particular message. This is an uppercase R: it differs from the r subcommand, which sends the reply to everyone who sent and received the message. When you give the R subcommand, follow it with the message number. For example:

? R 1
To:  cliflwr
Subject:  Re:  testing

Yes, I can make the meeting.  where ?
 ~.
EOT

The EOT indicates that your reply has been sent.