Undelivered Letters (First-Class Mail)
If the USPS is unable to deliver a letter, it will most likely be returned to sender. In the case of First-Class mail, the letter gets returned as soon as they discover it can not be delivered.
If you are the original sender of the letter, it will be returned to the return address. So if you have placed your complete and correct address in the return address area, your letter should be returned to you.
Keep in mind that First-Class mail does not include tracking information, so if you haven't received a response from your recipient(s) yet or they are saying they never received their letter, there really isn't a way to prove that's the case. It might be best to consider sending your documents via Certified Mail or FedEx so that you can track the delivery process and obtain proof of delivery.