The bulk of this post discusses IP reputation which is the biggest hurdle to self hosting. It is the first line defence for spam/phishing/fraudulent mail. SPF and DKIM can align for DMARC, but if the IP has a bad, or no reputation, chances are the message will be bounced. Think of it like having a low or no credit score. As an example, if a “cold” IP address suddenly starts sending mail, mail security will take the (arguably cynical) view that chances are it’s some sort of spam. It even more cynical to assume that this is done as a money grab, shepherding users to big mail services, when in fact, it’s the best way to prevent bulk mail.
The irony too is that, if you run a mailserver of substantial size, you will become very sympathetic to the other side on this. Dropping email from new/suspect IPs will tremendously reduce your user complaints about spam. It's not really a winnable situation.
