Getting a 'mailbox does not exist' message can be frustrating, we know. You simply need to give it thirty minutes.
Mailbox password changes, webmail access and sending/getting email by means of a mail client are for the most part accessible when your mailbox has been created.
However, it can take as long as 30 minutes for our MX servers to accept new email for your new mailbox. If you get a "Mailbox does not exist" bounceback to another mailbox inside 30 minutes of its setup, this ought to be nothing to stress over. It should resolve itself when our MX design updates.