The US Postal Service is working very hard for its customers. They are offering and providing them the best service they can. The mail carrier uses to deliver the mail and packages and if he/she finds no one at home, then he/she tries to deliver the same mail item on the next business day or leaves a PS Form 3849 (Delivery Notice). The post office holds undelivered mail for 15 days.
That means you have 15 days time for picking up your undelivered mail and if no-one comes to pick them up, then they are sent back to the sender after looking for the return address.
But what about those mail which doesn’t have a return address? These are known as Dead Mail. These type of mail are sent to Dead Mail Office where the workers try to find out the return address, and if they fail to find any return address, then they destroy all the mail items to protect the privacy of the customers. If there is anything valuable in the envelope, then they sell out those things at auction (except for firearms and pornography).
So, this happens to mail without return address.