While searching for something completely different, I stumbled upon paf's presentation at the marid-bof in February (http://www.imc.org/ietf-mxcomp/Seoul/PatrikFaltstrom/marid-bof.pdf, link also added on our homepage).

It contains both an clear list of mail flow cases (relaying, forwarding, etc), and observations like

: I am of the view people attack the spam problem from the wrong angle : : Look for a solution : Fine-tune it : Look for a problem the solution solves

Regards, /Niels