Until we have mail servers that are configured to verify their
configuration before sending each piece of outgoing mail [...]
Phillip, I'm honestly not being argumentative here. But what
exactly do you
mean by this? I think I don't understand.
I would expect that at some point mail servers that are trying to
send email from example.com that know they are external facing
would do a check of the DNS config to see that they are actually
configured to do this. Otherwise alert operator to fact that there
is a misconfiguration.