I’ve read that a good way to keep discussions civil and respectful is to talk about people as though they were right there in the room with you. In the information age, that’s a good rule of thumb to extend to email. If you mention someone in an email message—your friend, your teacher, your boss, etc.—pretend the email will be forwarded to the person.
“Mr. Meyers is an a**hole” becomes “I wish Mr. Meyers and I got along better.”
It doesn’t just insulate you from damages if your email were to get into the wrong hands; it changes your way of thinking.