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Re: [ietf-smtp] MTS-STS validation when MX host points to a CNAME, violating RFC 2181 § 10.3

2021-04-06 15:05:10
On 2021-04-06 10:16, Bron Gondwana wrote:

It's not an ideal world, but we don't live in an ideal world. We live in a real world, and in the real world "Microsoft are huge so they can handle the cost of doing what I want them to do" only works if you have a significant enough stick to incentivise them to do so.

I believe O365 clients of *paid* services could argue this is a breach of the contract. A client wants to deliver a message to user(_at_)example(_dot_)com. Sending MTA misleadingly says: Receiving MTA of user(_at_)example(_dot_)com has the problem A (MTA-STS validation failed), that's why we can't provide you a service you paid for. If that's not the case (and I suppose it's not: RFC8461, section 4.1 defines MX host Validation by matching MX record *name* against MTA-STS policy; the end).

Things are more complicated, I believe, and it depends on the jurisdiction(s), but when refusing to provide a paid service, I'd see the correct error reporting as a minimum requirement: by either showing generic or more specific errors, but never misleading ones.

That's more of a legal question though.

--
Regards,
Kristijonas

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