Your mail client was making a false assumption. That is a bug in the
software. The mail client shouldn't be looking up domains. It should be
sending it to the relay.
No, you're making an incorrect assumption. It's perfectly valid for a
mail client to send mail directly to the MX for the recipient domain
(or A record if there is no MX) That's how mail was intended to work,
having a local relay is just a popular optimization. So it's perfectly valid
for a mail client to look up domain names, and it's perfectly valid for a mail
client to assume that NXDOMAIN means that the domain does not exist. again,
that is how the mail protocol is defined to work.
Your attempt to reinterpret the mail specs in order to apologize for
VeriSign's fraud and unfair business practice is not in the least bit
persuasive.