“Who in their right mind would build an email client?” Gentry Underwood asked himself nearly two years ago. So he built an email client. “For a long time email was this area of tech that you didn’t touch, which was old and stodgy and not worth spending time on,” says Underwood. As Gmail hits its 10th anniversary, Mailbox has been quietly preparing its next big thing — a system that automatically clears out bothersome threads, spam, and emails you don’t need until later.