To reply to a message, you use the @reply command.
@reply to <number>
This command form will reply to the mail message number on the current mail list.
@reply <number> on <list>
This command form will reply to the mail message number on the specified mail list.
Either form of the @reply command will drop you into the editor to compose your reply. When you have finished your reply, just type 'send' and the message will be sent, and the editor will exit.