From a Wired News article:

According to recent research published in the Journal of Personality and Social Psychology, I've only a 50-50 chance of ascertaining the tone of any e-mail message. The study also shows that people think they've correctly interpreted the tone of e-mails they receive 90 percent of the time.

"That's how flame wars get started," says psychologist Nicholas Epley of the University of Chicago, who conducted the research with Justin Kruger of New York University. "People in our study were convinced they've accurately understood the tone of an e-mail message when in fact their odds are no better than chance," says Epley.