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So after the reception of an anonymous letter, I had a (rather whimsical) thought.
What about the creation of some sort of system that allows you to bribe the postmaster of a particular location, and have them reveal the last five people that have sent letters from their (the postmaster's) post office?
Suggesting five as the number to only give a list of possibilities, and not -entirely- defeat the purpose of 'anonymous' mailing.
Not entirely fleshed out, was more interested in overall feedback on the topic than details at this point (details would come later).