1. THE ACKNOWLEDGMENT RULE: Acknowledge in haste, respond at leisure. When you receive an email, acknowledge it within 24 hours if you can; take a week if you must, but more than that is (ordinarily) too long. An acknowledgment is not an answer. It's a one-liner, something like "thanks for your note; I'll be in touch soon." It tells the sender that his message has got through and that you plan to answer it some day. Once you've acknowledged a message, you should answer within (say) two weeks of sending the acknowledgment.

2. THE RESEND RULE: If an acknowledgment or (later) an answer doesn't arrive in good time, resend your message verbatim. The receiver's time limits dictate the sender's. If your message hasn't been acknowledged a week later, resend it. If the acknowledgment arrives but no answer has materialized two weeks after that, resend. So you get (at the outside) two chances to restart a sputtering conversation--and that's it. (When you resend a message, a discreet "2" or "3" in the subject line should be enough to let the receiver know what's going on.)

Where did the "24 hours, one week, two weeks" time limits come from? I just made them up. Maybe they're wrong. All I can say in their defense is that I've been a faithful emailer since 1982, and they strike me as about right.