What does rebuild mean in Apple Mail?

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Tags

1. #messages
2. #mailboxes
3. #stored
4. #downloaded
5. #issues
6. #exchange
7. #server
8. #attachments
9. #program
10. #rebuild
11. #emails
12. #discarded
13. #again
14. #resolve
15. #appearing
16. #accounts
17. #built
18. #search
19. #garbled
20. #computer