Reviewer: Yaron Sheffer Review result: Has Nits [Apologies for the late review.] * Intro: To avoid confusion, please mention the header parameter "No" to clarify why the header is named RequireTLS when its semantics is the exact opposite, "prioritize delivery over ability to negotiate TLS"? The same confusion already arises in the abstract. * Sec. 2: it seems to be implied that when REQUIRETLS is used, STARTTLS MUST also be sent. Is this the case? If so, please mention it (it's obvious to you but not to a first time reader). * I would have expected a parameter to be associated with REQUIRETLS to indicate whether DANE is required throughout the forwarding path, or MTA-STS, or either one will do. Although RFC 8461 takes care to not use the "opportunistic" word, it is in fact opportunistic to some degree (because an attacker can block the initial policy lookup with DNS) and so has different security properties than DANE. I am sure you've already beaten this horse to death, but if you have not, I think this is a real issue. The thing is, if ever we have widespread deployment of DANE (which I do NOT expect), this mechanism will not be as secure as it could have been. * Sec. 2 security requirements: the session must employ TLS: does this include pre-negotiation of TLS before starting SMTP, or only session upgrade with STARTTLS? This is common in Submission, I'm not sure about MTA-to-MTA use. * RequireTLS header field: I believe that the REQUIRETLS parameter MUST NOT be used when the header field is there, why not mention it? * If we define a case where MTA-STS policy is to be ignored (when using RequireTLS: No), shouldn't this document be marked as Updates RFC 8461? * The word "no" appears twice in Sec. 3, capitalized as "NO" or "No". Are parameters case insensitive? * The case of REQUIRETLS+RequireTLS on receipt is interesting, and the MAY requirement (Sec. 4.1) makes it non-deterministic. Why don't we simply disallow it and reject the message? * The textual name is mentioned twice, once with and once without a space. * 8.2 (active attacks): this solution is still vulnerable to the DNS blocking attack associated with MTA-STS (RFC 8461, Sec. 10.2), and this should be mentioned here.
