Not sure if you already solved this problem, but here we go.
You should take care of it using your DNS Zone File.
You should make an A record for mail to point to their Exchange Server (and they should probably update that server to use the new mail.juniperstreet.com.au). That's all. Leave the rest to point to your server's address and all works.
Edit: Actually, have a look here too: http://www.webdevforums.com/showthread.php?t=38550. Involves the same kind of thing.