Upcoming SMTP STS
My interpretation of SMTP STS:
New records of type TXT in the DNS are proposed instead of opportunistic mail exchange without a proper certificate.
Abstract, according to draft:
SMTP STS is a mechanism enabling mail service providers to declare
their ability to receive TLS-secured connections, to declare
particular methods for certificate validation, and to request sending
SMTP servers to report upon and/or refuse to deliver messages that
cannot be delivered securely.