"We spent a lot of time looking at the invite systems that other services like Gmail, Spotify, and Pinterest used to help them scale," Mailbox founder Gentry Underwood says, "and it became clear to us that we needed to do something transparent and equitable. It's the most honest way we can think of to get Mailbox into the hands of everybody who wants it."

One of the reasons Mailbox is so fast is because the company's servers compress your emails so they can reach your phone faster — and crunching emails for that many people isn't easy. "That's the basic technical challenge... Demand has been huge," says Underwood. "We've been hard at work trying to find a way to handle all the demand," he says. "This is email, after all — it has to work reliably."

Once you've signed up for the Mailbox wait list and downloaded the app (after it hits the App Store), a prompt will appear inside the app that shows how many people are ahead of you in line. When it's your turn, entering a code delivered by SMS will activate the app and let you start using it. It's unclear how long it will take for all wait list reservations to be fulfilled.

Along with announcing the wait list, Underwood commented for the first time about pricing options for Mailbox. "At launch Mailbox will be free, and our goal is to keep a version of Mailbox free of charge forever," Underwood says. "Like Evernote and Dropbox, we hope to add a premium set of paid features over time." Mailbox might choose to charge for adding more than one email account, similar to how Letterpress generates revenue, or it could try something new entirely.

While Mailbox's wait list approach may frustrate some, Underwood is convinced that it's far better to be safe than sorry. Honesty and reliability are obviously key features for an email service, and they're doubly important for an app like Mailbox, which pushes your email through its own servers before it reaches your phone. Hopefully a staggered launch will mean a successful launch — seeing Mailbox falter under the weight of demand on day one could scar its reputation forever.