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From https://tools.ietf.org/html/rfc3207:

  1. A publicly-referenced SMTP server MUST NOT require use of the
    STARTTLS extension in order to deliver mail locally.

  2. This rule (1.) 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.

  1. What does "deliver mail locally" mean in the first sentence? publicly-referenced SMTP server deliver locally ? What is it saying ?

  2. What does "damaging the interoperability of the Internet's SMTP infrastructure." mean ?

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In this context, "deliver mail locally" means that the mail server accepts mail for a domain and delivers it to a local mail spool from which the mail users download or read it.

And "damaging the interoperability of the Internet's SMTP infrastructure" means just that. If a mail server required STARTTLS then it would not be able to receive mail from servers which do not support STARTTLS. The RFC was released in 2002, and almost 20 years later there are still mail servers which don't support STARTTLS, either because they don't have the capability or (most commonly) because they are explicitly configured not to do so. It will probably be many more years before this could theoretically be required.

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