How different things are in the USA. Over the past thirty years there seems to have been a spate of killings by American postal workers, who have gone on the rampage and shot their colleagues, members of the public and police officers. Apparently official statistics suggest that the murder rate at post offices is lower than at other workplaces in the US.
Nevertheless – perhaps it’s just the strange idea of homicidal postmen – but for whatever reason the image gradually caught the American public’s imagination, and by late 1993 we begin to find the expression ‘go postal’ being used, meaning ‘to rush about in a murderous frenzy’.
It got a burst of publicity with its use in the 1995 film Clueless, which boosted its popularity still further, and it entered the general vocabulary of everyday American English, in the rather watered-down sense ‘become extremely angry’ (as in ‘Mom will go totally postal when she sees what you’ve done to the car’).
British speakers on the whole still treat the phrase as something slightly odd and unfamiliar, but don’t bet against it catching on here.