Postal has a number of uses. Normally just used as slang for "gone crazy" or can actually mean the act of going on a shooting spree. The term started after several postal employees went on shooting sprees.
Years after the term was no longer popular a company called "Running with scissors" made an EXTREMELY AWFUL pc game called "postal" which simply featured level after level of badly simpulated shooting sprees across a number of venues such as marching bands, gas stations, military bases, etc... Despite the fact that, had things been left well enough alone, nobody would have ever known this game existed a numvber of events happened to make this game 10 times more popular(but no less awful). First the U.S. Postal Service tried to sue the company over use of the name and failed quite horribly when it was proven that nowhere in the game was there even so much as a single refrence to any actual postal workers. Later, after the coulmbine shootings, Senator Liberman and a group of lazy parents tried to blame increased violence among children on the Video Game industry and Postal got even more free publicity when it was used as an example of violence in games.
What the US Postal Service, Senator Liberman and the fat lazy parents who hate video games failed to realise that their efforts only served to make Postal 10 times as popular as it otherwise ever would have been and nobody who ever actually played the game enjoyed it or tried to emulate it in any way.
Today video games are 10 times as violent and real than Postal ever was and gamers all over the earth hate Senator Liberman. However, anyone who claims to have seen me hanging around outside his house planning to go postal on his ass is nothing but a filthy rotten liar!