FORTUNE -- Not since the 1990s -- if ever -- have Internet users gone crazy over an email service the way they have for Mailbox for the iPhone. Some seemed more than happy to wait weeks in a virtual waiting list, one generated by the startup so its servers could handle the influx. ("I'm number 299,901!" opined one would-be user on Facebook.) Now just four months later, 36-year-old founder Gentry Underwood and his 15-strong team hope to replicate their initial success with this week's release of Mailbox for iPad.
<!-- ?xml version="1.0" encoding="UTF-8" standalone="no"? -->"There are products that are 'vitamins,' and there are products that are 'drugs,'" Underwood tells Fortune. "Vitamins are products where you think, 'Yeah, I should be using it in theory. Maybe I'll use it today.' A drug is something where you think, 'How the hell did I live without this thing?!'"
Given Mailbox's initial reception, Underwood considers the app a member of the latter category. As unexciting as a new email may sound in the Google (goog) Glass era, Mailbox's simple philosophy behind email on the go hit upon what Underwood calls a "resonance frequency." Although Mailbox does not disclose user numbers, there were 1.5 million sign-ups for the iPhone version before the digital queue was pulled offline mid-April. ("We're still taking on users at a really high rate," Underwood says.)
To recap, Mailbox offers a fairly unique, streamlined approach to mobile email. Power users need not apply here. Like the iPhone version, Mailbox for iPad uses simple gestures that treat messages like items on a to-do list: swiping to the left brings up a menu that allows one to basically hit the "snooze" button and choose when that message should pop up again as a reminder. A short swipe to the right checks an email off and places it in the inbox archive; a longer swipe to the right deletes it all together.
The iPad app offers virtually the same experience as its smaller sibling: the same onscreen layout, the same finger gestures to bring up the same options. The biggest draw? Answering email on a mobile device with a larger screen. Mailbox takes advantage of that but doesn't deviate much, if at all. That was purposeful: The Mailbox team wanted to quickly expand to the iPad without confusing loyal iPhone users by changing things up.
For Underwood, the past four months have been a roller coaster: seeing his product explode in popularity and behind the scenes, working around the clock to ensure the servers can handle the rapidly growing user base. That's a large part of the reason he decided to sell the startup to Dropbox for a reported $100 million. Having moved from their Palo Alto office to work from Dropbox's San Francisco headquarters, the team will take advantage of Dropbox's vast servers as it expands. Says Underwood: "We joined based on the hope that we'd be able to continue building Mailbox on user experience, to grow our own capacities without losing sight of the mission."