Going postal is an American English slang phrase referring to 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 1970 and 1997, more than 40 people were killed by then-current or former employees in at least 20 incidents of workplace rage. Between 1986 and 2011, workplace shootings happened roughly twice per year, with an average of 11.8 people killed per year.[1]

https://en.wikipedia.org/wiki/Going_postal