One from the "Letters of Note" category, and one particular brush that I had with such a thing...
Back in the 1990s, my wife worked in publishing. One of her gigs in Chicago was working for a scholastic press Open Court Publishing. One day while she and a co-worker were rummaging around in some cabinets trying to organize some files, they found an old, lost letter wedged in the very back of one particular drawer. It was a letter written in 1918, addressed to the then managing editor Paul Carus, who -- aside from being a noted German-American transcendenalist publisher and author in his day -- had also edited the company's journal of philosophy, The Monist. The letter was from the poet Ezra Pound, who was clearly displaced with Carus over the fate of a manuscript he'd submitted. My wife made a xerox of the thing before passing it along to be given to the Carus family.
The stationary bears the header for the London office of the literary publication The Little Review, and along its left-hand margin list the pub's contributors as: W. B. Yeats, Ford Madox Hueffer, Arthur Symons, Wyndham Lewis, James Joyce, T. S. Elliot, Lady Gregory, Arthur Waley, May Sinclair, "Jh.", Margaret Anderson (Editor), and Ezra Pound (Foreign Editor).
The text of the letter, as follows:
La Salle Ill. U.S.A
Jourdain makes some faint excuse for your continued incivility and your putrid meanness in not returning my mss. of the Fenollosa essay on the Chinese Written Character. (To be sent to J. Quinn 31 Nassau St. New York.)
Jourdain says that you are supposed to be ill. I hope you are. and what is more I hope you die of it.
In the mean time return my mss. and crawl out of the thief category, and make your peace with whatever diseased deity is provided for such baccilli as yourself.
Damn you again, and three boils for your infected liver.
As it turned out, Carus was in fact gravely ill, and died of his illness some 8 months later.