Using email cuts down on phone calls, but sometimes relying on it, can turn a simple exchange into a week of back and forth conversation. First of all, emails messages should be short and easy to understand.
It is sometimes hard to determine when it is better to either pick up the phone or walk to someone's desk. Even if they sit 50 yards away from you, it can sometimes save you a ton of time and aggravation. (It's also a nice diversion).
Many people are multi-tasking and only giving their email correspondence a portion of their attention span. This creates extra work for themselves and those they are emailing with. If you find that someone is giving you half answers or creating confusion, it's time to talk.
Also, if there is a personal issue or potential conflict, there should be a verbal conversation. And if you have an item that helps explain your communication, visit the person's desk.