Thank you for using the Jiskha Homework Help Forum. In computer language (e-mails, etc.) CAPS are considered impolite = like shouting. Here are some sites on tone and emails:
6. (good advice): http://www.booherbanter.typepad.com/booherblog/2006/11/your_emails_des.html
NOTE: To me, the vocabulary is important to determine tone. It's true with e-mails you have no body language, pitch, etc. to help you understand.