Here's a fail safe way to not lose any email when moving hosts. These instructions will show you how to prevent your users from missing any email while your transfer their accounts over to Joyent.
user_a@servername.joyent.us and user_b@servername.joyent.us.user_a@servername.joyent.us.user_a@mydomain.com ⇒ user_a@servername.joyent.us. At this point, there should be no new mail arriving at your old host into your old email boxes. It should all be forwarded to the mailboxes on the textdrive servers, regardless of DNS settings (old or new)And that's it, you should have now safely transferred your email accounts over to Joyent.