Transferring email accounts from another host

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.

Make sure you have enabled mail for your domain, otherwise your user will not be able to send or receive email. You can enable mail from Virtualmin by doing the following:

Sign in to Virtualmin (see Account logins and important URLs).
Select a virtual server from the drop-down in the left-hand navigation (your main virtual server should be already selected when you first login to Virtualmin).
Click Edit Virtual Server in the left-hand navigation.
Click Enabled features
Select Mail for domain enabled? and click Save Virtual Server

Set the TTL on your DNS records low, like 30 seconds or so, and allow a few days for that change to propagate over the internet.
Add the appropriate domains to your Joyent account, and create a new POP3/IMAP email account for each of your users, which will give them an account like user_a@servername.joyent.us and user_b@servername.joyent.us.
Set up all the mail destinations on your Joyent account so that all the @mydomain.com emails go to the right mailboxes
Teach each email user to access this new account via either webmail or by adding a new account to their email program (e.g., Outlook). At this stage, they should be able to access both the new and old email boxes, and should be able to receive a test email at user_a@servername.joyent.us.
Edit the mail set-up on your old host so that all the email forwards to these new email boxes, e.g., 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)
Now you can switch your MX records and DNS records to point to the Joyent servers, raise your DNS TTL, effectively completing the migration
After a day or two, you can close the old hosting account and do whatever you want

And that's it, you should have now safely transferred your email accounts over to Joyent.