A few years ago in my second "real" job, I got tangled up in an ego-mail pissing contest that a few people were CCd on. Afterward a friend (a director at the company with more professional experience than I) imparted to me the following advice:
When you write a directed email under any level of stress at all, read it to yourself and add "..., you asshole." at the end of every sentence. If the tone of the resultant does not change at all, you need a rewrite.
That little piece of advice has saved my butt on numerous occasions since then.