God just open them and read them!
This happened to me and after mail kept coming despite being redirected I opened it and found out it was from the scientologists.
It was addressed to the former resident who was a high ranking member of the church.
The letters detailed their plans to build a spaceship and establish a colony in space when the end of the world comes.
Part of the plan, according to the information gleaned from the letters, was to brainwash local residents into pledging allegiance to a dastardly plot to launch a string of massive saurin gas attacks on capital cities around the globe, bringing about said end of the world and said rise to power of the Thetan race.
Following the leads in the letters I was able to infiltrate their organsiation, blow the whole sordid plan wide open in an award winning scoop and play a central role in the prosecution and conviction of the ringleaders.
mmmwahahahahaha
This is all made up, but my point is, you never know what you might find out...