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Scorpion451
(Edited uphill both ways)

Character A decides to vent their frustrations over Character B by writing a pretend letter that's basically a "The Reason You Suck" Speech, with the intent of never mailing it (either by destroying it or locking it away somewhere). Unfortunately, due to a twist of fate or two, the letter ends up being mailed anyway. Hilarity Ensues as Character A tries to intercept the letter before Character B can read it. Modern versions often replace the letter with an e-mail or voicemail message.