Milters
This section covers the basics of how Milters works
Concepts
Email RBLs (real-time block-hole lists) Primer
https://kb.mailchimp.com/delivery/spam-filters/how-blacklists-work
https://www.pinpointe.com/blog/how-do-i-know-if-im-on-a-spam-blacklist
https://rbltracker.com/blog/2015/07/what-are-rbls-and-how-do-they-work-part-1/
https://blog.returnpath.com/blacklist-basics-the-top-email-blacklists-you-need-to-know-v2/