At this point, replying is the absolute stupidest move you can possibly make. There are precious few things in life that can't be ruined to a magnificent degree with an ululating phone call or a hasty "Oh, you think I suck on a professional level? That is curiously similar to what I told your mom last night" reply to the unpleasant email from the boss. The important thing is to remember these simple things:
First, step away from the keyboard. Seriously, back the fuck down. Right now, there's nothing you can do with that thing that will not get your ass in trouble. Wait at least an hour before replying, it'll give your brain time to sand the edges off your rage. To take your mind off the topic for an hour or so, maybe pop on over to the nearest sports store and idly pick up some random items. A ski mask, for example. I mean, obviously that's just one off the top of my head, but yeah, pick it up. Shopping can be very therapeutic; it'll soothe you.
Go to a spa. Or, just repeatedly say the word "spa."
If at all possible, reply with a phone call instead of emailing back. It's noticeably easier to keep your calm when you're actually talking with a fellow human being. If you think you're still too pissed off for that, you can try typing up the angriest reply you can possibly muster from the bile of your hate-filled system -- just do it on a notepad or something, so you can refrain from actually sending it. This should help you clear your head and assess the situation calmly.
It didn't? Shit. I was afraid that might happen. Still, before you click "send," at least take some time to ...