I worked in the Mail Operations of the Church for 6 years, but it's been 15 years ago, so some things might have changed. Our operation was run by a former post master of a nearby large city, so he kind of knew the ins and outs at that time.

It's important that you black out the address, any bar code USPS put on the letter front and back. Often people don't notice that usually on the back of the envelope there is another bar code that needs to be blacked out as well. It's usually in a florescent orange, so not as noticeable as black bar coding on the front. This will make it so the letter won't be resent back to you again. Anything though that isn't 1st class mail (generally marked "presort standard" or "bulk" mail, the USPS will dispose of, so it doesn't do any good to try to return it.