It's worth also mentioning MTA-STS [1], currently a draft. It provides a mechanism for domains to declare a STARTTLS requirement via a file at https://<domain>/.well-known/mta-sts.txt. It's backwards compatible and opt-in. There seems to be some testing already going on [2].
[1] https://tools.ietf.org/html/draft-ietf-uta-mta-sts-21
[2] https://mta-sts.gmail.com/.well-known/mta-sts.txt