Want to make a quick hundred mil? Apparently all you need to do nowadays is design an app. It worked for Gentry Underwood, creator of the Mailbox app for iOS. Underwood was tired of the limitations he encountered when using his iPhone email. You have to admit, Apple’s mail app has its weaknesses. Compared to programs such as Microsoft Outlook, it only lets users look at one email at a time, and it’s limited in the actions available to the user. So Underwood set out to build a better mail app.

“We use our mobile phones really differently than we use our laptops,” Underwood recently told Wired. “It’s taken us a while to figure that out and understand it, and that creates a design opportunity for a different kind of tool.”

Underwood’s Mailbox app, which just launched on a limited basis in March, turned out to be so ingenious—it provides users new ways to organize and clear and inbox—Dropbox was already willing to pay big bucks to acquire the technology. Underwood just sold the email organizer, from which users can swipe a message to “snooze,” archive, delete or file it, for $100 million.

“We actually were two-and-a-half years old when we sold,” Underwood explained. “But still, without commenting on the specific numbers of the deal, I will say that this is a very unusual market and the potential impact of these devices, I think, is what’s driving it. Mobile is ripping the world wide open. The companies that respond to that stand to be quite relevant in the future, so that’s what creates these seemingly crazy numbers.”

But what’s so special about mailbox that makes it worth that kind of dough when its marketability has barely been tested?

“Mail itself is this very old protocol that generally is very slow,” Underewood said. “To deliver mail quickly when you open your device, we actually took a lot of infrastructure that historically has lived directly on the mail client and we moved it into the cloud. When you set up a mailbox account, Mailbox begins checking your mail from the cloud, reformatting it, sending you push messages when there are new messages, and having this tiny little snapshot ready for you when you open up the phone that, as soon as you hit the network, we just hand it to you as quickly as possible. That allows us to create a fast experience even though we’re only accessing the radio for a brief amount of time.”

So there you have it. Proof once again that if you build a better mousetrap, the world will beat a path to your door.