Don't Be A Novelist
Messages should be concise and to the point. Think of it as a telephone conversation, except you are typing instead of speaking. Nobody has ever won a Pulitzer Prize for a telephone conversation nor will they win one for an e-mail message. 

Its also important to remember that some people receive hundreds of e-mail messages a day (yes, there are such people), so the last thing they want to see is a message from someone who thinks he/she is the next Dickens. 

Too Much Punctuation!!!
Don't get caught up in grammar and punctuation, especially excessive punctuation. You'll see lots of e-mail messages where people put a dozen exclamation points at the end of a sentence for added emphasis. Big deal. Exclamation points (called "bangs" in computer circles) are just another form of ending a sentence. 

If something is important it should be reflected in your text, not in your punctuation.

The Legacy Of Punch Cards
Although this is the 1990's, not everyone in the world has e-mail software that has the word wrap feature (word wrap keeps you from having to hit the Enter key at the end of the line). There are still a large number of users with dumb (and not so dumb) terminals and teletype devices that do not gracefully handle text longer than the old punch card length of 80 characters. Therefore, keep the number of characters per line below the 80 character limit. Some recent e-mail packages have a built-in feature that automatically word wraps at a specified character limit so that the problem is essentially solved. However, if you're software does not support this feature, you'll just have to remember to use the big Enter key again.

Formatting Is Not Everything
Formatting can be everything, but not here. Plain text is it. Period. End of sentence. 

Using HTML to format messages so that they have fancy fonts, colors or whatever is asking for trouble. There are lots of e-mail clients (and some servers) which can not handle messages in these formats. The message will come in as utter gibberish or in the worst case, crash the e-mail client. 

Abbreviations
Abbreviation usage is quite rampant with e-mail. In the quest to save keystrokes, users have traded clarity for confusion (unless you understand the abbreviations). Some of the more common abbreviations are listed in the table below. I would recommend that you use abbreviations that are already common to the English language, such as FYI and BTW. Beyond that, you run the risk of confusing your recipient.

Smilies
Part of the nature of a good one-on-one conversation is the use of visual cues. How important are facial expressions and body gestures to a conversation? A simple eye movement can mean the difference between "yes" and "YES". What about auditory cues? The results are the same. 

Since there are no visual or auditory cues with e-mail, users have come up with something called "smilies". They are simple strings of characters that are interspersed in the e-mail text to convey the writer's emotions (cues). The most common example is :-). Turn your head to the left and you should see a happy face (the colon are the eyes, the dash is the nose and the parentheses is the mouth). 

Salutations
The question here is "How personal is too personal?" or to be more specific, how do you open your e-mail: "Dear Sir", "Dear Mr. Smith", "Joe" or none of the afore-mentioned. 

In a non-business situation, it is okay to bypass the standard formalities. In the business situation, things are much more complicated. Each situation will need to be evaluated on its on. If you are unsure, stick to the formal salutation. It's the safest bet.

Compiled from web resources