Avoid All-Capital Words and Abbreviations
Capital letters should be reserved for the beginning of sentences and proper nouns. Capital letters are akin to shouting on the web. It is not seen as a polite thing to do, and certainly not professional.
The same rule applies for abbreviations. They do not make for easy reading. Some abbreviations (like 'lol' for 'Laughing Out Loud') are considered juvenile and are not suitable at all for a professional environment. Likewise for smileys and emoticons, which are best avoided in a professional environment. You may, of course, use them when the recipient is someone you are very close to.