A couple of days after sending a really angry email to one of our maintenence service providers my boss came over to me with a print-out of it. He said "Next time you feel like this, take an extra coffee break, come back and reassess the situation. I doubt you will feel the same way." Its pretty smart advice that I have to follow quite alot. Writing angry emails isn't as satisfying as yelling or punching someone so you can tend to go a little bit overboard on the tone and language if you're not careful.