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.
“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.”