[danmarg]

If you required TLS on all SMTP, you would in fact end up having to fail a large number of messages.

Even worse, of the domains that support STARTTLS, a sizable number either don't present certificates that chain to a widely trusted root, or don't present certificates that actually match their MX. Worse still, because many domains' MXs don't match the domain itself, even if the certificate is trusted for the MX, it may not be trusted for the domain.[1]

So I think unfortunately we're not anywhere near a world where we could actually just drop email on the floor if TLS-with-a-valid-cert isn't present ("valid" not being clearly defined here, of course). I do think we're slowly moving in that direction[2].

It's certainly true that retrofitting security makes the whole thing more complicated, of course. That's a strong argument for ensuring that any retrofitting we do is itself forward-compatible with what we want to do in 30 years. Or for inventing time machines.

1. http://static.googleusercontent.com/media/research.google.co... 2. e.g. http://arstechnica.com/information-technology/2016/02/gmail-...

[stonogo]

>Even worse, of the domains that support STARTTLS, a sizable number either don't present certificates that chain to a widely trusted root, or don't present certificates that actually match their MX. Worse still, because many domains' MXs don't match the domain itself, even if the certificate is trusted for the MX, it may not be trusted for the domain.

This is madness, though. TLS is transport-layer security. It's not kerberos and it was never intended to be. The cert produced by the MX should be valid for the MX. Trust is an illusion, but to the extent that you decide to trust anything in the CA system, you trust it for the MX only and use SPF etc to determine if the MX is the correct one.

STARTTLS is a bad idea and should go away entirely. It forces an SMTP server to care about the transport layer, and that's entirely incorrect. I have plenty of SMTP servers that communicate on my networks without ever touching TCP/IP -- forcing them to support STARTTLS specifically is moving backwards.

This RFC is strongly dependent on the internet looking pretty much exactly like it looks today, and enforcing that mode of operation indefinitely. It's short-sighted and harmful to the entire email world.

[danmarg]

But I think the authentication problem is in fact the hard problem. Assuming we got rid of STARTTLS (the actual verb) and just always did TLS (say, on some other port), how do you propose to solve it?

[stonogo]

Fortunately, we have an extensible protocol that already supports service advertising and negotiation. There's no reason we can't have an AUTH module that works both ways (both the client and server mutually authenticate, independent of the transport-layer encryption).