Sendgrid puts a huge amount of money and resources into maintaining both technical and political legitimacy. They have relationships with other major senders and receivers of email, they follow every single rule to the T, they aggressively police their own IPs (of which they have many) etc. It's a complicated business, which is why it's totally worth it to pay them to do it for you.
Yes there is a real lesson about the pragmatic impossibility of true decentralization in email. Specialization of labor is a useful thing, it turns out.
That being said: the fact that is a protocol and not a proprietary standard does have decentralizing effects that do provide a useful incentive check against providers, even if there are only a few of them. There is also a real lesson there too, that one I'll leave for the reader.
