letter bomb 
noun
An explosive device hidden in a small package and sent to someone with the intention of harming or killing them: [as modifier]: letter bomb attacks...
- On the letter bomb front, army bomb disposal experts were called on to disarm a letter bomb sent to an unnamed agricultural business and a farm.
- The identity of the American Unabomber was assisted by the analysis of saliva left on a letter bomb he sent that did not explode.
- Four weeks ago, a letter bomb addressed to Mr Attwood was sent to the Belfast headquarters of the Policing Board.