[uiri]

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.

[tptacek]

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.

[phicoh]

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.