Posting my email reply:
When you go through your email for inbox zero, are you replying to each individual email that requires a reply, or otherwise doing stuff for each email? Because you can just move it to a folder and add the stuff you have to do to a list, GTD-style. That way you can get to inbox zero faster. The point of inbox zero is not necessarily to complete all email-related tasks, just to list them so you don’t have unknown stuff in your email. A lot of your suggestions have the problem that they hide possibly urgent emails.