Only one month after the email app Mailbox was released to a glowing reception, the company was purchased by Dropbox. The email service has received great reviews from users, and was delivering 50 million messages a day a mere three weeks after launch.

At the 2013 TechCrunch Disrupt conference in New York City this week, Mailbox CEO Gentry Underwood spoke about his app's big launch, the company's approach to solving problems and his relationship with Dropbox since the acquisition.

As Mailbox was in development and connections with Twitter were in the midst of being developed, Underwood said that what he wanted to do was create something that would become really successful.

"I really wanted to build a product that gained traction," that really took off, he said, but it wasn't clear Mailbox was going to get there. "We launched the product and there was a groundswell of interest at the right time, which caused some VC [venture capital] interest ... When it rains it pours, and suddenly everyone wanted to do a deal."

So while the timing may have simply been a "right place, right time" kind of event, developing the app was anything but. The creation process at Mailbox is very design-oriented, which could be a surprising thing to hear for an email app, but Underwood said design may not mean what many people think it does.

"When people hear the word design, they think about making the product pretty," he said. But to Underwood, the term design means solving problems. It's about locking onto difficulties and figuring out how to make them better. The design process is to iterate quickly and make sure you can fail as much as possible before you run out of money. You fail until you succeed.

"Don't be afraid to question your assumptions and kill your darlings" in order to move forward, he concluded.

According to Underwood, an engineer knows if you're iterating a solution and you come up with another problem, you've failed. You need to narrow down that problem even before you start engineering the solution.

Generally, though, he tries to bake optimism into the culture; to ask, "How can we solve issues?" rather than thinking, "We can never solve this."

When it came to reconciling his own vision for Mailbox within the larger goals of Dropbox, Underwood said it has been a surprisingly smooth process to continue operating Mailbox the way he wants to.

"What we found since we've been there [Dropbox] is it's exactly what we get to do." His team had many ideas concerning how Dropbox and Mailbox could integrate and work together, but they had to pick out the most important ideas. Instead of dictating terms, Dropbox simply asked Underwood "What's best for the user?" That's exactly the type of approach Mailbox wants to take going forward.