This is a translation of my previous post Etiqueta en el citado de mensajes de e-mail.
Introduction
The most ignored Internet Etiquette rule is that of the posting style when replying to an e-mail message.
By «quoting» I mean the inclusion of a previous message in a new one (for example, a reply) with the intent of stating the relevance of the answer itself in the original context. I would say «Dan, I don’t know what are you talking about. Please quote the original e-mail in your reply next time in order to know what were you replying to.»
Almost every e-mail client do quote the original message in the reply, at least by default. That is good, and it contributes to the «netiquette», but its abuse has made it completely useless.