You're focused on a task when you suddenly see a blinking alert on your desktop that informs you that you have a new email. Although you resolved to stick to your work, your mind can't help wandering and you start mulling over what the email could be about. More often than not, the email can wait an hour or two, and disrupting your "in the zone" mode just decreases productivity and will end up prolonging the time you take to finish up a project.
To be more productive, switch off the notification alerts on your email and set aside an hour or two during the day that's dedicated to emails. You can split up the times in chunks and check and respond to emails in the morning, after lunch, and late afternoon.