jail mail

(名詞) 監獄囚犯寫給報章雜誌編輯的信函。

The letter from inmate #374155 had lain on my desk, buried amid piles of correspondence and research for a few days. I get a lot of "jail mail" and I know what to expect.
—Fannie Flono, "From inmate 374155: Don't drop out," Charlotte Observer, October 1, 2010

In prisons across the country, with their artificial pre-Internet worlds where magazines are one of the few connections to the outside and handwritten correspondence is the primary form of communication, the art of the pen-to-paper letter to the editor is thriving. Magazine editors see so much of it that they have even coined a term for these letters: jail mail.
—Jeremy W. Peters, "The Handwritten Letter, an Art All but Lost, Thrives in Prison," The New York Times, January 7, 2011