The mail service has a remarkable ability to track mail packages, even those not sent by certified or registered mail. Additionally, police stations log all calls received--there's no such thing as an anonymous call. The use of a pay phone, prepaid cell phone, or online burner number is a huge red flag--it supports the recipient's defense that someone is attempting to ruin their reputation. Plus, almost all child pornography cases are coordinated with a special DOJ task force, due to the international scope of this offense.
Once it's been established that the recipient is being framed, the police--and the FBI--turn their attention to finding the actual perpetrator. The use of the postal service makes the frame-up a federal crime.
This is actually how the FBI actually cracked one of the more infamous child porn rings a few years ago--some idiot tried to frame his neighbor over some stupid dispute and they traced it back to him quite easily.
The classic Japanese serial novel Musashi ( http://www.amazon.com/Musashi-Epic-Novel-Samurai-Era-ebook/d... ) includes a fairly disturbing subplot about the protagonist's reputation being trashed by a little old lady of no significance who follows him around insulting him to anyone who will listen. Despite the fact that no one knows her, and many significant and influential people know (and like) him, this prevents him from landing a respectable job.
Kind of like destroying someone's reputation via sybil attacks, on an app like lulu or yelp or whatever. The NSA had slides on how fake victims could write blogs about being raped or mistreated or whatever.