When Mailbox was first announced last month it received a lot of attention. Nearly every major tech blog covered the app and the company's vision. The app was so well received that Mailbox may not actually be able to handle the amount of users that will potentially download the app when it is released. Mailbox is now offering a way for keen users to reserve their access to the app.
The main way and in which Mailbox is so fast is that it downloads your emails to their servers, compresses it, then sends it over to your iPhone. Because they're a small company they don't have the server capacity to handle the potential user base which will download the app.
To reserve your spot, head over to the Mailbox site and enter in your mobile number. When the app is released you'll (hopefully) receive a text which will give you a code. You put this code into the app to get access.
Of course it's first-come-first-serve. So the sooner you sign up the better.