Email is not this wonderful, democratic system of equals. Email is more like a school playground where a handful of enormous bullies control everything. You either learn to work with them, cater to their demands, or you'll never be able to visit large sections of the playground.

Gmail is relatively easy to appease, mostly just configuring your nameservers and email headers to Google's standards, then slowly building "reputation". Hotmail will block an entire domain (IP?) randomly every few years, then you fill out one of their infamous forms and pray they unblock you quickly. Yahoo (technically Verizon now) is even more difficult to work with, they block you out of nowhere and I've never figured out how to appeal. Seems like they do some kind of ratelimiting for smaller email servers, I don't know if it's some kind of punishment or just by default.

Then there's the dozen other major email providers. Some easy to work with, others not so much.