A rejection letter needs to do two things: 1) softly inform the recipient that their work will not be accepted and 2) kindly encourage them to do better next time. Hunter S. Thompson has the appropriate verbosity and temperament to pen such letters. And he fulfills both criteria in a rejection letter for Rolling Stone to some author named Anthony Burgess.
"What kind of lame, half-mad bullshit are you trying to sneak over on us? When Rolling Stone asks for 'a thinkpiece,' goddamnit, we want a fucking Thinkpiece ... and don't try to weasel out with any of your limey bullshit about a "50,000 word novella about the condition humaine, etc.... Do you take us for a gang of brainless lizards? Rich hoodlums? Dilettante thugs? You lazy cocksucker. I want that Thinkpiece on my desk by Labor Day."