SMTP Strict Transport Security

> You still need a way of knowing whether the server supports the new security protocol,

Thats where the reference to STS comes in, you add a new capability flag to EHLO that signifies that all further communication with that server should happen over TLS directly, so the first connection caches that information and all subsequent connections can't be downgraded because they aren't relying on checking for a cleartext STARTTLS flag every time. Of course if the first connection you make strips this info, then you can be downgraded, just like with SSH or other protocols that cache security relevant information on first connection, but that's a different kind of risk.

Over time you can switch clients to TLS by default and will see fewer and fewer clients who need the hinting or require the fall back cleartext protocol, giving you a real sense of when you can safely sunset to old protocol.