I've read the article several times. :-)

OK, to use your scheme I have to stop accepting
connections directly to my port 25 and accept them
only from "trusted servers". That's essentially
the same thing that I have in mind. With your scheme,
I can send email myself, but only through a trusted
server (at least if I am sending to someone wanting
to avoid spam and who thus only accepts mail from trusted
servers), and I should accept connections also only from
trusted servers (unless I want to get spam again). Under
my scheme, it's essentially the same: I send through a
trusted server (provided by my dynamic-DNS provider) and,
if I drop connections from non-trusted servers, essentially
also accept email only from trusted servers. Not much
difference really.

The only difference is that, under your scheme, an unknown
could send an email into the MX network and it would
eventually get through. In my scheme, he could do so
with no penalty if he sent it through a trusted server.

Do you know of anyone running such a throttling server who
invites everyone to send email through it? If I had the
resources to run such a server, I would rather relay email
from people I know to be trustworthy (and who contribute to
the cost) rather than accepting from anyone and throttling
unknowns (most of which are spammers).

In both schemes, to stop spam computers have to stop accepting
port 25 connections from "just anywhere" and accept them only
from trusted servers. In both schemes, email has to be sent
through trusted servers (one which will relay legitimate email
immediately in my case and not even accept other connections,
one which will relay known senders quickly and unknowns slowly
in your case). In practice, my scheme already works for those
who want to use it. Do you know of anyone running these
throttling servers?

I think your scheme would work well if it existed, but do you
see any way for it to come about? My scheme can be built up
piece by piece. More and more people will stop accepting
connections from arbitrary addresses to stop spam, and accept
them from trusted servers. There is a motivation to send email
through trusted servers, and a market and competition. For example,
dynaccess.com offers a "reasonable" number of emails as part of a
flat fee (and it is possible to register special sender addresses
for sending newsletters etc) with access based on IP address, whereas
dyndns.org offers a micropayment-per-email scheme using
authentication.