Well, yes, of course. If you work for an at-will employer (and many of us do, at least here in the US), then you can be fired at any time, for pretty much any reason. Bad e-mail etiquette is not a protected exclusion in the eyes of the court.
And the reality is that people have been so fired. Last year, someone named Vicki Walker was famously dismissed for a pattern of sending e-mail in all caps (and, apparently because that wasn't loud enough, for using red text) to co-workers.
Luckily for Vicki, she lived in New Zealand, where she sued her former employer for wrongful termination and won a judgment of $17,000. Would she have won the case in the US? That's hard to say -- "at will" employment policies don't always protect employers as thoroughly as they often hope.
Word to the wise: What made this particular case so ambiguous was the lack of any corporate e-mail policy. Walker had no official guidelines telling her such e-mails were unacceptable, and her manager had no yardstick by which to measure her behavior. And that, as we saw, was a recipe for anger, frustration, dismissal, and lawsuits.