What is Mxtoolbox blacklist?

Related questions

1. What is MXToolbox?
2. Can a phone be removed from blacklist?
3. What is the purpose of MX records?
4. How do I find my incoming mail server host name?
5. Can someone find your address from your email?
6. What is Registerdns?
7. How do I look up DNS?
8. Can you trace an email from Gmail?
9. Can I have multiple MX records?
10. Can you trace Gmail IP address?

Tags

1. #blocking
2. #realtime
3. #sending
4. #domain
5. #respectively
6. #prevent
7. #server
8. #problem
9. #email
10. #never
11. #delivered
12. #blacklist
13. #dnsbl
14. #known
15. #blacklisted