Macro payments through domain names?

Hi everyone,

This list has previously discussed micropayments for emails. I
recently had a related idea which I haven't seen discussed. How about
using the cost of buying top level domains as the base of more of a
macro payment system?

Here we go:
Put the hash (eg., md5) of a public pgp/gpg key used for signing
emails in a DNS entry derivable from the email address. Enforce the
DNS lookup to be in two steps. The first lookup returns the reference
to an external top level domain; the 'eMail Authorization Domain'
(mad). The second lookup is a TXT query towards this 'mad' which must
return the hash of the public key of the email signature (where the
public key and the signature data is included in the end of the email
as usual).

Now use a system of blacklists similar to how it works today, but for
sent spam, blacklist the involved 'mad' instead of the domain of the
sending email address. This makes a 'mad' used for spamming
"unusable" for further emailing, and someone will have to pay for a
new domain that isn't yet on the blacklists; but when doing so one
will be able to keep using the same email address as before (which is
important if some other user under your domain was responsible for
the spamming).

Spammers paying for their own 'mad's will have a hard time to make
profit, and will be traceable by whois data. Spammers using hacked
systems will really push people towards higher computer security
since such incidents now leads to a real monetary cost. However, the
actual cost will hardly drive anyone out of business, since it can
be limited to the registration fee of one domain per organization.

I guess it is also possible to rework this scheme into micropayments
by registrating the 'mad' of every received (and properly signed)
email with a blacklist, and then block the 'mad' after say 10'000
emails. But it is not obvious to me why that would be much better
than the above.

Any thoughts?
//Rickard