https://news.ycombinator.com/item?id=19756125 just saw this again today on HN. Google refusing mails from a mail provider without giving any meaningful error message or reason.
We had similar issues with microsoft and our #zombi mailserver (whole hetzner ip space was blacklisted there)
E-Mail is federated in itself but as there are large "instances" like google and microsoft they can make it feel like a centralized service and make it very hard to run independent instances by dropping their messages.
Mastodon Instance of the zom.bi Community