Talking to a colleague at work we ended up talking about inbox overload and he confessed receiving 100 emails per day, out of which maybe 20 require answers and 5 require long and detailed answers.
Now imagine this. Assuming you actually work 8 hours per day (no toilet breaks; lunch? You're too fat anyway!) and receive 100 e-mails, you will have (again assuming no meetings during that day) 4 minutes and 48 seconds to devote to each e-mail. Not bad you say. Not so fast...
Since 20 e-mails require answers and assuming you are quite fast and are able to read and understand the e-mail in 2 minutes and answer it in 1 minute you will have read 75 emails (100-20-5) in 150 minutes (75*2 min to read) and answered 20 e-mails in 60 minutes (20*(2 min read + 1 min to answer)).
So, you've spent 210 minutes handling 95 of the 100 e-mails that you got, that's a whopping: 3,5 hours. Half your day was just used reading and answering 95% of your e-mails, but wait, you have left the best for last, you still have 5 e-mails that will take you some real time to answer.
The long and detailed e-mails that require answers that we often have to search for can take up to 20-30 minutes to answer each. Let's be optimistic and assign those e-mails an average of 15 minutes to read and reply to each. Fifteen minutes times five e-mails mean that you will spend a full 1h 15m reading and answering those last 5 e-mails. This gives us a grand total of 4h 45min for e-mails per day!
But that is not too bad, right? You still have 3h 15min for real work! Well, not quite. You see, those e-mails arrive at random intervals and they take your concentration away from other important work that you should be doing, therefore they actually have a much bigger impact on your performance. Every time you switch from doing something to reading/answering e-mails you loose time trying to catch up with what you were doing before, that's the task switching overhead.
Not answering e-mails is not the solution. Talking to people directly is the solution. If someone sends you an e-mail with a non-trivial question don't even bother trying to reply in writing immediately. Call the person or better yet, walk over to the person and talk to her, the 15 minutes you spend talking to her will be much more productive than the 15 minutes you would spend answering her e-mail.