The fundamental source of poor email style is the practice of quoting the entire message you’re replying to. If that’s what you do, then it doesn’t matter whether you put your response at the top or bottom. In fact, if you’re going to quote the entire message, top-posting probably is better. But both are poor form.
Writing an email is like writing an article. Only quote the relevant parts, interspersing your new remarks between the quoted passages. Don’t quote anything at all from the original message if you don’t have to.
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