Go to your source email system (the one you're migrating from), and navigate to the list of mailboxes you want to migrate. ...

Go to your source email system (the one you're migrating from), and navigate to the list of mailboxes you want to migrate. We'd give you the exact steps if we could, but there are so many different email systems out there that you need to find out these steps on your own. When you find the list of mailboxes, keep the window open so you can refer to them.