Encourage people to push their correspondence with you down the distraction chain:
- Phone / Face-to-face
- Instant messaging
- Email
You can do this by deffering them: "I'm really busy right now, can you send me it in an email?"
This should reduce the amount of interruptions you receive allowing you to stay "in the zone" for longer periods of time, increasing your productivity.
Finally, allot time for processing emails at set times of the day. I, for example, have my email set to send and receive once every two hours. This bulking of activities allows you to get more done in the day without impacting customer relations.