[dsr_]

RFC2487 says:

A publicly-referenced SMTP server MUST NOT require use of the STARTTLS extension in order to deliver mail locally. This rule prevents the STARTTLS extension from damaging the interoperability of the Internet's SMTP infrastructure. A publicly-referenced SMTP server is an SMTP server which runs on port 25 of an Internet host listed in the MX record (or A record if an MX record is not present) for the domain name on the right hand side of an Internet mail address.

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However, this RFC is now 18 years old.

https://datatracker.ietf.org/wg/uta/documents/

shows that https://datatracker.ietf.org/doc/draft-ietf-uta-email-deep/ is now in Last Call -- the title is "Cleartext Considered Obsolete: Use of TLS for Email Submission and Access" and coming up soon we have https://datatracker.ietf.org/doc/draft-ietf-uta-mta-sts/ with this abstract:

SMTP Mail Transfer Agent Strict Transport Security (MTA-STS) is a mechanism enabling mail service providers to declare their ability to receive Transport Layer Security (TLS) secure SMTP connections, and to specify whether sending SMTP servers should refuse to deliver to MX hosts that do not offer TLS with a trusted server certificate