SMTP TLS Reporting, first data
The SMTP standard is just as ubiquitous as it is ossified and hard to change. Thus all newer RFCs follow similar patterns of a) adding optional extensions and b) providing feedback loops to detect usage, problems, and abuse of these extensions.
One more recent extension tries to enforce TLS transport between mailservers. This sounds very simple (and would be very simple if one could change the SMTP standard to require TLS), but is not trivial when compatibility is necessary and all TLS policies are “opt-in”.

