‘Going Postal’ means to become extremely angry, often to the point of rage and violence, especially when provoked in a workplace setting.
Example of use: “Did you hear that Mark went postal yesterday at the office, after his manager told him that Jane’s getting the promotion he was promised.”
The origin of the phrase derives from a series of incidents involving U.S. Postal workers, who shot and killed fellow workers and members of the public in 1986. The first recorded use was in an article by Karl Vick's 'Violence at work tied to loss of esteem', published on December 17th 1993 in the St. Petersburg Times: "The symposium was sponsored by the U.S. Postal Service, which has been so many outbursts that in some circles excessive stress is known as 'going postal'..."