A publicly-referenced SMTP server MUST NOT require use of the
STARTTLS extension in order to deliver mail locally.
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.
What does "deliver mail locally" mean in the first sentence? publicly-referenced SMTP server deliver locally ? What is it saying ?
What does "damaging the interoperability of the Internet's SMTP infrastructure." mean ?