originally coined from a series of real life shootings in the postal service, it now usually means that someone is about to go nuts or off the deep end. the reason for going postal is usually trivial. also, means person on psychiatric meds that is off their pills.
after finding her computer's wallpaper had been changed again, Jane went postal on her fellow workers.
Basicly, "going postal" is pulling a uzi, assault rifle or most any other firearm out from under your jacket and going nuts. That is, wasting anyone and anything you see, and then probably yourself. A bit like going apeshit.
Jim had left another burning paper bag of crap on John's doorstep, so John retaliated by going postal on his ass.