A convicted murderer who gunned down his wife and her boyfriend on a busy Queens street wants to go back to court — over his lost mail.

Masao Yonamine, 67, claims he sent 6,500 pages of legal documents in two boxes from his Otisville, NY, state-prison cell to a woman in New Jersey, shelling out $35.20 in postage.

But the box never arrived, his Manhattan federal court suit states.

He says losing the documents caused him “to lose all hope or confidence to obtain his liberty.”

The US Postal Service is at fault for “negligently loosing [sic]” the boxes of documents, which contained “substantial exculpatory evidences,” Yonamine claims in court papers.

The cabbie-turned-killer, acting as his own lawyer, says he never got the return receipt he requested on the certified mail, and he wants $90,000 in damages.

On Dec. 4, 1986, Yonamine used his taxi to run his wife’s car off of Queens Boulevard after the two fought in Family Court over custody of their three kids.

Yonamine got out of his cab and began shooting in broad daylight, striking his wife, Alice, through the windshield of her car. He also shot her boyfriend, John Cote, in the head as he attempted to flee.