The Babe of Business has an excellent entry about email etiquette.

In this day of Instant Messaging and Texting too many people are using those shortcuts in emails and other communications. While that may be fine if you are talking to one of your friends, when communicating in business, you should your proper business communication.

The last suggestion she poses is actually one I would put closer to the top although it is one of the last things you need to do and that is to check who the mail is being sent to. This is especially important if it's dealing with confidential information and data or something you do not want the rest of the world to find out.

She has a total of twelve items to look for. I would also recommend that all small business owners have a book on proper business communications. While I need to find a more updated version, I find myself referencing it frequently to check out the proper format for a certain type of letter or other important information.