Of course, they could have gone with DNSSEC, but HTTPS has the virtue of solving the same problem (modulo proper configuration), while also being more widely deployed and better understood.

As for "better understood," the authors of this standard work for Google, Verizon, Microsoft, and Comcast. Combined they probably serve most of the email in the western hemisphere.

There is no excuse for overcomplicating a proposed standard -- and "we have this stuff lying around" is no exception. This plan has the effect of causing a networking protocol to depend on an entire series of unrelated networking protocols.

It does not bode well for the future of independent email operators and it provides no path forward for integrating existing devices. It pushes many, many extra steps onto client devices, necessitating entire additional libraries of software to deal with them. That's not a problem for companies with tens of thousands of developers at their beck and call -- but it is only going to raise the barrier-to-entry of operating an independent service.