Am I missing something, this e-mail was "in house" to an IT team basically? Without knowing what kind of a system it is, did you accidentally e-mail clients and harm the companys reputation?
In that case I can understand a backlash and even being fired, telling everyone at work, accidentally, what you think of a system that most people also believe is crap is nothing, I wouldn't make the slightest deal out of it.
If I was the boss I'd be asking why the system is so crap and finding out how whoever was put incharge came up with a crap system. Its an e-mail, why should anyone get in trouble over essentially nothing, not least because it was a joke and accidentally sent to people it shouldn't have been.
I had the classic thing at work years ago like in Office space if anyones seen it, company with three directors, I made ONE mistake in 6 months, I was on crap pay and doing FAR more than my job title entailed, I was happy too, I worked hard from the second I walked in the door to the second I left. One mistake that(internet order company) meant one order needed to be shipped again, and I got crap from 3 different bosses. IT would mean essentially no profit on a single order but probably no loss, out of thousands and thousands of orders.
Mistakes happen, thats life, no one should ever ever get as others have said, made to feel inferior by anyone. It shouldn't happen in school but thats another matter, it certainly shouldn't happen in the work place.