Email has been replaced by social networks as the de facto time waster but that doesn’t mean we're using email any more efficiently. It’s amazing for some things: memorializing the details of a meeting, sharing the details of plan, exchanging information, describing goals. Notice the focus on quantitative exchange.
Here’s what email sucks at: creative dialog. Both words are important there: Creative, meaning there's subjectivity and lots of influences at work and dialog, because you anticipate going back and forth. If you ever feel the need to start brainstorming via email consider stopping and actually talking to someone.
Here’s a rule of thumb that I try to follow (as with all rules of thumb it’s flexible): if an email is more than 50 words consider having a conversation, either face-to-face or on the phone.