But there are even people that still self host email server (have a look in the selfhosted subreddit for example). IP reputation is a thing, for sure, but I don't feel that it's been brought up by the big corp wickedly, it's a good way to prevent spam to arrive to the server. There are thousands of email providers in the world that are not Google, Amazon, Microsoft or some other big corp. This means that is possible. Is it difficult? For me for sure!!! But I think that the rising difficulty has been a result of this fields over the years. Just my 2 cents.