This story might have more to it than meets the eye:

A Wisconsin state appeals court refused to overturn the conviction of a prisoner who said no one ever saw him write a threatening letter to a judge.

The 2nd District Appeals Court noted Wednesday that Anthony Dwane Turner’s return address was on the envelope, sealing its decision.

Since he was a prisoner at the time, it would seem his return address was determined for him:

Turner asked the appeals court in 2005 to overturn his conviction. He argued prosecutors didn’t present any evidence at trial that anyone saw him write the letter or that the handwriting was his.

The appeals court refused, saying the envelope was marked with Turner’s name, inmate number and the address of the prison in Green Bay.

So I guess the question raised is whether someone in a prison can impersonate a prisoner by sending a letter that appears to be from their cell. The address is treated as a signature, but how trustworthy/spoofable is it?