I thought "The 13th Letter" was a pleasant surprise, a good mystery that keeps you guessing until the end of the picture. In hindsight, it seems a good idea to place the film in Quebec - it seemed a little quirky when the film began but you gradually get used to it. Acting was good all around, especially the two old pros, Michael Rennie and Charles Boyer. Director Preminger uses all cast members as suspects and sprinkles suspicion around liberally - so much so that you can't pinpoint the culprit until late in the movie.
As noted in the site's summary, someone is writing poison pen letters to the new doctor in town (Rennie), and copying in various and sundry townspeople. The letters accuse the doctor of an affair with the wife of the head of the local hospital (Boyer). This is a well done mystery which I can't recall as being on TV. I think it would be well-received if it were dusted off by FMC - it was produced by 20th Century Fox and must be in their vaults somewhere.