To achieve 0 emails in your inbox. I believe it means to have "bounced up against zero emails". When you achieve Zero Email Bounce, your inbox is not growing out of control, with unhandled stuff. When you achieve Zero Email Bounce your stress level goes down.
I achieved zero email bounce within the first two days after I started using the "Getting Things Done" inbox-management method.
by Odiwan April 3, 2006