The phrase "to leak the letter" means to secretly and intentionally release or disclose a letter or its contents to the public without permission. Full definition
A recent leaked letter from the BRDC chairman saying Silverstone was looking into activating a break clause in its contract cast the future of the British GP into doubt. (wtf1.com)
Leaked letter shows Lib Dem Simon Hughes taking Chris Grayling, the justice secretary, to task over removal of check - off facility for departmental employees (theguardian.com)
And December brought us another nasty shock: leaked letters revealed that in 2015, Theresa May had wanted to use the Immigration Bill to require schools to check children's immigration status, and push the children of undocumented migrants to the back of the queue for school places. (schoolsabc.net)