Why do MX records take so long to update?

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Tags

1. #email
2. #directed
3. #account
4. #administrator
5. #through
6. #records
7. #gmail
8. #domain
9. #accounts
10. #during
11. #change
12. #hours
13. #update
14. #suite
15. #might
16. #bounce
17. #system
18. #setting