How did deliverability become a discipline?

Deliverability became its own specialty in the early 2000s when mailbox providers started using content filters, reputation systems, and user spam complaints to decide what reached the inbox.

A key development was the Feedback Loop where providers like Hotmail and later others would send complaint data back to senders whenever a recipient clicked This is spam. This gave senders real signals to clean lists and improve practices.

It was like harbor masters finally sending reports back to fleet owners so they could stop sending cargo to ports that did not want it.