Going Postal" has come to mean losing one's temper.
It turns out that 20 years ago a postal worker, Patrick Henry Sherrill went on a shooting rampage systematically killing 14 people, then committed suicide.
I guess he delivered lots of postcards but only received bills. That would push anyone over the edge.
From the article:
"Postal workers are no more likely to resort to workplace violence than workers in other jobs. A commission on workplace violence found 0.26 workplace homicides per 100,000 postal workers from 1992 to 1998. By comparison the rate was 2.10 per 100,000 for retail workers, 1.66 in public administration, 1.32 for transportation and 0.50 for private delivery services."
I can see why retail workers would go postal. Talk about a thankless job for no money.