What happens to undeliverable snail mail that has no return address?

Question

A misaddressed postcard, for example.

Answer 1

It goes to what is called the "dead letter" office, and eventually is destroyed.

Answer 2

put in a pile labled undelivered mail and people can come in and claim it later on. I was once told this by some one in my family, not sure if it is true.

Answer 3

Comes back as cat litter!