In the US, there was a spate of postal workers going, well, postal, a few decades back.
Quoth Wiki: Going postal is an English slang term, used as a verb meaning to suddenly become extremely and uncontrollably angry, possibly to the point of violence. The term 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. Between 1986 and 1997, more than 40 people were killed in at least 20 incidents of workplace rage. Following this series of events, the idiom entered common parlance and has been applied to murders committed by employees in acts of workplace rage, irrespective of the employer; and generally to describe fits of rage, though not necessarily at the level of murder, in or outside the workplace. http://en.wikipedia.org/wiki/Going_postal
Also, the US Post Office *does* maintain its own police force: http://en.wikipedia.org/wiki/United_States_Postal_Inspection_Service