No specific source for the purpose. That's just classic mafia/oligopoly playbook. A more explicit version would be: "that's a nice email server you got there, would be a shame if your mail were silently dropped; If you just pay for my services that certainly wouldn't happen"
Of course, they can't do that with other big players (like Gmail) so they're allowlisted, because they would be afraid of lawsuits and their users would complain about not receiving email from Gmail. By applying this to smaller providers only, they maintain plausible deniability that the problem lies on the other side, a sentiment which is reinforced by maintaining their tech support's lower levels unaware of the scheme (the only ones you can talk to for months after initial complaint).
Agree. The accusation is that microsoft is dropping its competitors' mail despite their reputation. If the reputation is there, there's no other motive than to force consumers to use microshit.
Citation for this? We have a witness in the room! WhyNotHugo's comment is a primary source.
Cite?