Since proper email transport encryption is finally getting some momentum with DANE and MTA-STS, I'm surprised that STARTTLS Everywhere doesn't mention them.
DANE is being recommended (mandated?) by the European Union, and is on the rise. MTA-STS is backed by Gmail, Microsoft and many others, which is likely to give it traction.
To sum up from that post, we think STARTTLS Everywhere is a stop-gap measure until DNSSEC is fully deployed, and STARTTLS Everywhere can act as a preload list for MTA-STS (to prevent DNS downgrade attacks).
DANE is being recommended (mandated?) by the European Union, and is on the rise. MTA-STS is backed by Gmail, Microsoft and many others, which is likely to give it traction.