Mailbox is one of the best Gmail apps for iOS we've ever seen. Recently acquired by Dropbox, Mailbox's super clean design and gesture-based interface make it easy to sort through clutter and master your inbox. You can quickly archive emails, "snooze" messages to save for later, and turn emails into to-do lists. We chatted with co-founder and CEO Gentry Underwood to find out how Mailbox came to be.