Mail providers like Mailgun live or die based on their reputation. If they as a sender get marked as spam by "The Algorithms", then their product is useless. They almost certainly respond to abuse reports quickly and have enough reputation, and enough legitimate clients, that they have "leeway". But that leeway isn't unlimited.

There was a big fit thrown on this website a year or two ago when one transactional mail provider (MailChimp?) said they were banning crypto companies with ICOs from their service. They literally put "ICOs are not welcome" in their Terms of Service. Why? Because first off, ICOs are scams. And that means they buy email lists, and then spam the shit out of people with "buy into our ICO!" emails, which get immediately shitcanned into the spam folder by 99.99% of users -- they're no different than Nigerian Princes, as far as most people care. That behavior tanks the reputation of the sender, and you cannot reason with The Algorithm once it "recognizes" you as spam. That kind of problem is not a minor inconvenience to companies like Mailgun, it's an existential threat.