The model is like a simple C/R involving Message ID of a mail. The
sending domain provides the Message ID and allows computational time
for the receiving domain and then query the sending domain to send
the message.
This model has been proposed many, many, times before. I suppose we should
make a page for it in the ASRG wiki of anti-spam techniques even though it's
more of a mail system redesign than an anti-spam measure.
R's,
John
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