> If your choice in any way depends on the configuration of the mail server then your decentralized system has accomplished absolutely nothing, because the mail provider is still the gatekeeper.

This is a strawman. If you are talking about a decentralized system, you don't need the mail provider to send the message for you. Your email application would (and should) be able to resolve the IP address of the destination server for you, and it would (and should) be able to verify that the end server is who the sender expects it to be.

So, what would happen is that if you are on ENS', your view of the network would follow the consensus established by the ENS' chain. If you try to send a message to an email server that is on ENS, they would fail to provide a valid confirmation that they can respond for the IP address that you think they are on. It would be akin to failing a DNSSEC query.