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.
to succumb to fatigue and tension, to be become violently angry(from a series of incidents from the mid 80s to the 90s where various postal workers went on shooting rampages)
The United States Postal Service- A government organization responsible for receiving and delivering mail within the United States and also exporting it to other countries for international delivery. Very slow.
Hey! I thought you sent me that package?
Yup. I put it throught the postal service! Wait a few days.