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STS is built around cached policies. It does not require an HTTPS transaction per message. Since a very high fraction of all email is sent between the sponsoring organizations (Google, Yahoo, Microsoft, and the ISPs you mentioned) it is likely that the aggregate cache hit rate will be approximately 100%.

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If so much mail is sent between the sponsoring organisations that the aggregate hit rate will be approximately 100%, then wouldn't approximately 100% of mail be sent over the TLS only port were the sponsoring organisations to decide to only use TLS among themselves (e.g. a hardcoded SMTPS everywhere list) ?

The point of an open standard is so that the vanishingly small fraction of people running their own mail servers can easily implement it too.

Yes, this is an instance where the giant email providers could simply have agreed among themselves not to allow SMTP TLS MITM attackers, and then left everyone to fend for themselves.

Maybe it is just me, but I get mail from lots of small organisations. Maybe the bulk volume of mail is handled by a few parties, but if I look at all mail I care about, there are lots of parties.

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