First thing in the morning I check and answer email. Then throughout the day. Then at night after dinner. Then before going to bed. It's not that I am addicted, it's that my company spans from the Middle East west to Asia, and either a customer or someone in my organization is always up and working.
But the hard part of email isn't answering it, it's not answering it, which is important in several cases:
Cooling Off - I will admit that I am really bad about this, but sometimes waiting to tell off that VP could wait until the next morning when you cool down a little. Execs don't have all the information, history and background on a problem, and when they state a solution that I think is wrong, I usually answer straight away why they are stupid and my solution is superior. This can be okay if stated politically, which isn't the case when I dash off my first thoughts like "what are you thinking, you moron?" Cooling down and writing an email that is more politically savvy is a better idea.
Waiting on Someone Else - Sometimes I get an email from a customer asking about something that I have already tasked someone else to do. Sometimes a "we are working on it" email is okay, but sometimes just waiting another day and sending an email with all the info is better. Sort of a judgment call.
Pacing - Negotiations take time and sometimes you should think about an email for a day or two before replying, or just pace the negotiation appropriately by waiting to reply. Right now I have a large Asian customer that sent me an email last Friday asking me if I would reply to a pricing request by this Wednesday. I didn't reply to that email and will get back to them on the pricing this Friday. They are on the hook and aren't going anywhere (if you think of this situation like dating, it will make total sense).
Political - I have read that you should wait x hours before replying to certain emails to let people think you are busy. I am not into that, but I will admit that sometimes not replying is better politics than replying, depending on circumstances. Also, not answering or delaying an answer can say as much as replying in some instances.
Excuses - And one of the best reasons not to answer email: "Oh, you asked for xyz? I must not have seen that email..."