Graphic designers frequently use capital letters in advertisements and poster designs. The capital letters might bring your attention to a particular product or set a style for the design. Used in this manner, the public does not take any offense to the capital letters. They are just letters in the upper case.
When capital letters are used in e-mails, it is considered shouting and is very rude. People associate an emotion with these capital letters. It is also very difficult to read capital letters in an e-mail.
Unless you want to be shunned or cause a ruckus, please restrain from using all capital letters in an e-mail message.