To handle the influx of potential users, startup Orchestra employs a virtual line, allowing new users onto their servers in controlled bursts. Sign up for an account, and you’re placed in a reservation queue. It may be a convenient process for the company, but it’s also an excellent marketing ploy. Would-be users — like the one who sent us the screen grab on the right — reveal that that queue has reached epic proportions and have taken to social media to talk about the wait with a fervor normally reserved for hardware launches. “I’m number 299,901!” opined one.

Founder Gentry Underwood tells Fortune his app is seeing impressive traction already with 750,000 total sign-ups and 30,000 new users being activated daily. Even for the Internet, that kind of take up is pretty spectacular for an app rolled out just four weeks ago. “Scaling is really hard,” Underwood explains with a tired-sounding chuckle. Mailbox now processes 8 million emails daily.

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Mailbox offers a new twist on mobile email. Power users accustomed to Gmail’s message folders and color coding won’t find that. Instead, they’ll find simple gestures that treat messages like items on a to-do list. A finger swipe to the left on a message 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: Later Today, This Evening, or This Weekend. A short swipe to the right checks an email off and places it in the archive; a longer swipe to the right deletes it.

Email chains are streamlined to take up less space so users can more quickly scroll through them. Individual emails are downloaded and uploaded quicker than via popular competitors like Apple (AAPL) Mail or Sparrow thanks to some math on the back-end that compresses messages to take up less space.

The early success must feel good for Underwood, whose previous venture, a desktop and mobile task organizer called Orchestra, won Apple’s Productivity App of the Year in 2011 but quickly lost users. Underwood believes they became disillusioned with or intimidated by the idea of a comprehensive to-do app over the long run. What people were doing, he says, was treating email more and more like task reminders, sending themselves lists or treating individual messages as tasks. So the Orchestra crew did an about-face and baked some of what they had learned from Orchestra into the all-new Mailbox app. After using it for several days, I must say it is easy to use. It amplifies a sense of accomplishment once one’s email count reaches zero.

It’s too early to tell whether Mailbox users will stick around once the excitement dies down, but Underwood and crew have proven that there’s opportunity .