That sucks! And fixing reputation is at best a nightmare. I've seen suggestions about purchasing / sending email from other domains - to protect your primary domain. Not something I really care to do.
Google themselves recommends sending different types of emails from different subdomains of the primary domain to help Google differentiate between transactional, marketing, newsletters, outbound, etc
Any half decent marketer will 100% use a different domain for outbound sales (or any use case where spam rate might be abnormally high).
I'm not nearly as sure. The mechanics of one aren't that hard to imagine. Encourage spammers and make it cheap. Don't ever fight them at their source. Invoke mechanisms that intentionally destroy public utility in public protocols. Force everyone to rely on a small handful of "reputable" senders.
Working backwards from "who decides reputation anyways?" might make it easier to see.
Sending requires quite a large volume for the big players to allow you to play. The only viable option for small servers is to use a SMTP relay service. Amazon's was a pain to get out of the sandbox mode but has been reliable and most importantly free.
As I care more about the recieving side than sending emails this works well enough for me.
Any half decent marketer will 100% use a different domain for outbound sales (or any use case where spam rate might be abnormally high).