What happens to dead letter mail?

Related questions

1. How do I schedule a pickup from USPS?
2. How do I put my mail on hold?
3. What is a mail stop?
4. Can USPS hold my package?
5. Can I pick up my USPS package early?
6. Can I pick up my package from USPS distribution center?
7. What does arrived at hub mean for USPS?
8. Does the USPS photograph all mail?
9. Does the post office open mail?
10. What does intercepted mean on USPS tracking?

Tags

1. #office
2. #invalid
3. #system
4. #within
5. #cannot
6. #letter
7. #facility
8. #sender
9. #return
10. #considered
11. #returned
12. #delivered
13. #there
14. #processed
15. #undeliverable
16. #postal
17. #address
18. #where
19. #addressee