Mind Hacks has an interesting article on why email is addictive. The basic theory is that it has "variable interval reinforcement". We keep on clicking the Send/Receive button or checking emails because sometimes when we do that we see an email from a friend or something important we want to action right away or something funny. We keep on checking because sometime we will be rewarded. Just like the person on the slot machines keeps on going, because there is the belief that sooner or later they probably will win something. Checking emails becomes an automated response following on from that reward expectation deep down, even if we know in our logical brains we should be doing something else, or the chance of anything new coming in is slim.