So far, I used the rawest approach possible, by sending tens of thousands of mails via PHP's internal mail function. When I send one to mail-tester.com, I get

So far, I used the rawest approach possible, by sending tens of thousands of mails via PHP's internal mail function. When I send one to mail-tester.com, I get 9/10. Seems pretty good? I don't see why Google should care for technicalities. They can see the behavior of their users. The behavior should tell them that the mails are not spam.