The Collaborative International Dictionary of English v.0.48:

letter bomb \let"ter bomb`\, letter-bomb \let"ter-bomb`\n.
   A bomb disguised as a letter and sent through the mail,
   usually rigged to explode and kill or harm the recipient when
   opened.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

letter bomb
    n 1: a thin explosive device inside an envelope or package and
         detonated when opened [syn: letter bomb, parcel bomb,
         package bomb]
    v 1: send an explosive to; "The Unabomber letter bombed a number
         of individuals and institutions"