It can prove a lot - i know case of one very famous singer that package was addressed to his dog name, and he get sentence of probation (1 year or something like that - EDIT: I mean this singer, not his dog )
It happened in one of the EU countries
he probably had an idiot lawyer or he ran his stupid mouth
noone has power over what anyone sends you per mail, trust me i know what im talking about
It depends what can be proven or not, and different countries have different laws and enforcement of their laws. I just send to a false name with regular mail so it can't be traced back to me and the receiver can always denies it's his/hers and when such a envelope gets lost I just send a new one. Also I send only to trusted family and friends and not random people so the chance they're going to rat me out is small.