A more drastic measure that you may need to consider if you're reeaally behind is to declare email bankruptcy like this guy did:
Lawrence Lessig hit upon a novel tactic after spending 80 hours trying to clear out his backlogged inbox: surrender. "Bankruptcy is now my only option," he wrote in a mass message to his correspondence creditors. Here's how Lessig erased his debts and turned over a new leaf.How about you? Any fresh starts in the new year? Feel free to share in the comments.
1) Collect the email addresses of everyone you haven't replied to. Paste them into the BCC field of a new message you'll send to yourself.
2) Write a polite note explaining your predicament. Apologize profusely - Lessig managed five mea culpas in as many paragraphs - and promise to keep up with your email in the future. Try to sound credible.
3) Ask for a resend of anything particularly pressing, and offer to give such messages special attention.