What is Going Postal?

Legal Definition
Going postal, in American English slang, means becoming extremely and uncontrollably angry, often to the point of violence, and usually in a workplace environment.

The expression derives from a series of incidents from 1986 onward in which United States Postal Service (USPS) workers shot and killed managers, fellow workers, and members of the police or general public in acts of mass murder. Between 1986 and 1997, more than 40 people were gunned down by current or former employees in at least 20 incidents of workplace rage.

In Australia, the phrase is used in reference to the Queen Street massacre in 1987, in which gunman Frank Vitkovic murdered eight people inside an Australian Post office building in Melbourne.
-- Wikipedia