In a report of several studies of email use, Psyblog.com presents compelling reasons to limit email use:
1) every time we open an email, we need at least a minute to get our minds back to what we were working on previously
2) we open email far more frequently than we believe we do, with most people checking email every 5 minutes.
The report says we spend 23% of our work day checking email, or 110 minutes in an 8-hour work day. I'm not sure how many discreet emails this assumes, so I can't estimate how many minutes we spend getting our minds back into the task we were focused on before we opened email. However, I'm betting it's close to an hour.
Use email as a effectiveness and efficiency tool, not as a drain on your time and energy, by setting your email notification to 45 minutes rather than 5 (or immediate!) and disciplining yourself to check email only then.
Let me know how this works for you!