The real productivity killer is not email, twitter, facebook or checking the Dow Jones every five minutes per se. Getting bad news (e.g. an angry client message, a new competitor, a lost project bid, etc) is what really kills your productivity.
A single bad news can easily block me for the next two hours. I know I should have more self-control but once you get the news it’s very difficult to stop thinking/swearing about it until somehow you “digest” it one or two hours later.
If you need to get something done soon, block the outside world but not because it’s terrible to relax by reading emails from time to time but just because between those emails you can find a hidden bad news that will drain your energy for the rest of the day!!!
I’d love to have an email filter that only showed me nice emails until those periods of the day when I’m ready to face the harsh reality of the entrepreneurial life!