I think we're not so far apart on the technical side. I don't
see the "smells like extortion" problem. You seem to be thinking
of this AOL scheme where the SENDER could pay to have an email
bypass a spam filter. I agree with you here: that is wrong, wrong,
wrong. What I'm thinking of is that the RECEIVER can decide (if
he wants to; no-one forces him to) if he wants to accept from only
trusted SMTP servers. If he runs his own email server (as you and
I do), this is entirely within his control and can be managed by a
RBL (at no cost). If he doesn't run his own email server, this is
a service his ISP could offer. Perhaps the ISP would charge for it.
If the customer thinks he charges too much, then he can move to a new
ISP. Free market. The ISP might even charge LESS since he can just
drop such SMTP connections without having to forward the mail to
the customer, i.e. a business model "pay if you want everything, pay
less if you want to accept mail only from trusted SMTP servers".

Again, this is happening already. Soon after I set up my stuff
at home, some emails bounced (most of them to academic addresses).
It turns out that they were rejecting emails from dial-up IP
addresses (probably via a RBL). For a SMALL additional fee (we are
talking about a few dollars a year, much less than I pay for my
DSL connection per month (which costs me EUR 30 per month for 16 Mb/s flatrate) my dynamic-DNS provider lets me send outgoing email through
his SMTP relay server. Now, I let most of the spam through (I do
reject a few IP addresses and From: addresses which have spammed me
several times, as well as dropping all connections to non-existent
usernames (dictionary-attack spam)), but since the volume has picked
up recently, I plan to be more aggressive in filtering. For a small
fee I could run all INCOMING mail through my dynamic-DNS provider,
who would scan it and tag it as spam based on content (and some other
criteria); I could then filter these out at my end (spending less
time on them the more I'm convinced that I have set the proper degree
of strictness for the spam tagging). Another alternative would be
to continue to receive stuff directly, but drop more connections,
specifically those from "bad" IP addresses. Actually, although I
might have an additional white list, it would primarily be a black
list, perhaps using publicly available RBLs.

With a) many people using volatile IP addresses and b) much spam
coming from virus-infected PCs, it seems that the strategy of
accepting email only from trusted SMTP servers is the only way to
a) cheaply and b) efficiently fight spam which is c) relatively easy
for everyone to use.

Free speech? That doesn't cover shouting "fire" in a crowded theatre,
right? While there might always be an issue, the outcome of the
issue is not always that "everything is allowed since it is free
speech". Maybe the laws are different in Europe than in North
America. In Europe it is illegal to place unsolicited commercial
telephone calls or send faxes (i.e. telemarketing). Spam is
essentially similar. Legally, it is the same. Practically, it is
easy to trace a telephone or fax number and press charges.

Again, this is not that much different than throttling email from
unknown addresses and letting the stuff from known good guys through
quickly. However, with the trusted-SMTP-server concept, everyone
who runs a server (an end user, or his ISP if he doesn't run his
own mail server) can decide for himself which servers are trusted
and a customer can move to an ISP with a different concept. If
email is slowed down, it's slowed down, and this is a decision
made by the person running the server which the customer can't
influence. That is less control in the hands of the end user, so
I'm surprised you prefer that concept. My concept is "pay to be on
the white list". Since the payment is a) voluntary, b) small, c)
might even be NEGATIVE since it means less work for the ISP (i.e.
you would get a discount for accepting only mail the ISP thinks is
OK) and d) much, much less than other internet-connection costs,
I don't really see the payment as an issue at all, especially since
no-one is forced to do it. (That is, no-one is forced to do it
with respect to receiving mail. You can still run your own mail
server and do what you want. With respect to sending mail, you might
get rejected by some people, but again that is happening already.)