What does the Post Office do with undeliverable mail?

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Tags

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