'Shouting' in e-mail terms is when you use capital letters in your correspondence. We all receive e-mails that are full of capitals urging us to respond immediately. LAST CHANCE, ONE AND ONLY OPPORTUNITY, etc. It's designed to catch your eye and make you respond quickly. The same principle applies to those people who indicate that every e-mail they send is urgent. Both of these antics tend to irritate people who treat them as urgent and needing attention, only to find that they are direct sales pitches.
The use of abbreviations in an e-mail is equally frustrating, especially if you have no idea what the abbreviations mean. Corresponding by e-mail is quicker than writing a letter or sending a fax, but the same rules of etiquette apply. All e-mails should be correctly addressed, names should be spelled correctly, and the body of the message should be well written and proofread carefully. If it isn't urgent, don't mark it as urgent.
As time goes on we will all utilize e-mail more and more as a fast, efficient and cost-effective way to communicate, but when it comes to offering good levels of customer service, be careful of being overly casual and informal when contacting people, especially if you don't know them very well.