Looking at the U in UBE

This is apropos of nothing much but maybe it'll find a sympathetic ear on this
list.  I signed up years ago and activity has been so low that I have other
projects and little time to spend on thinking about email protocols.

That said, It seems to me that decent sender authentication would open up many
candidate solutions.  Like this one:

An email costs the sender $0.50 to be placed in the receiver's inbox (or, more
strictly, for the sender to be notified of its presence).  When the recipient
reads it, he/she decides whether it was wanted or not.  If so, $0.499 is
refunded.  If not, the sender loses his/her 50 cents.  MUAs can allow
recipients to configure a list of senders who are always refunded.

If the church wants to send out a mailshot, it can send 1000 emails with a
single dollar.  But it'll need say a $5 buffer to allow 50 mails to be sent. 
the rest would sit in the outgoing queue until refunds started arriving.

The 0.1 cent per email levy is to contribute toward infrastructure costs and
give ISPs a reason to bother.

This is predicated on
1) Senders' bank details being made known
2) Certification of senders
3) more formal relationships between serious service providers

It gives you
1) Wanted mail is almost free to send
2) Unwanted mail is much less cheap to send
3) Spamming as we currently know it will either need a lot of financial backing
or will involve serious felonies like defrauding a bank.
4) Mailing list owners will have more of an interest in pruning their databases