Email etiquette is necessary, especially when communicating with professors, colleagues, and your boss. Although as a society we've shorted words and phrases to mere letters (lol - laughing out loud, ttyl - talk to you later, or wdteg - where did the elephant go), they have no place in an email. Emails appear to be informal, but that informal email can be forwarded to every single person with an account at the click of a button. Therefore, you need to make sure it is well composed. Who wants to be known for bad writing on an already scandalous email (I'm just assuming it's scandalous if it's good enough to be forwarded to everyone)?
When writing an email, start with a greeting. It does not need to be super formal, but here are a few examples:
Professors - "Dr. Smith," or "Prof. Smith," if you are unsure if they have a doctorate.
Boss - "Mrs. Courtney,"
Colleague - "WASSUP CASEY!" Okay, bad example. Try, "Hey Casey,"
Then include a body of well-constructed sentences. Example:
"I was reviewing my notes from your Monday and Wednesday American literature class, and I still don't understand why Ginsberg was HOWLing?"
Make sure to use capitalization and punctuation. Also, use the spell check. Nothing looks worse than a person who doesn't spend the extra 10 seconds to double check their work.
And always end with your name:
Before you hit send, read your email again. These guidelines are simple and easy to follow, but they'll take you a long way. You can talk to your friends however you want, but when you're in a professional setting, you want to be professional.
Purdue's OWL (online writing lab) has a great page with tons of information concerning other aspects of email etiquette.