For the same reason you would never SHOUT down the phone at a customer, you should never type an email in UPPER CASE. But what else do you do that might be turning good customers away?
As a web designer, I see a lot of emails. And some of the most badly written emails I’ve seen are from merchants. Many of them are far too busy to give the niceties of email a first thought.
But at what cost?
Read the article – How to retain customers through proper use of email