p.s. At some point lawmakers might decide that a client's failure to
honor NO SOLICITATION is a kind of denial-of-service attack, but I sort
of doubt it will ever get to that point. I consider it somewhat more
likely that they would require senders to include Solicitation: fields
in their messages, and that they would define specific keywords to use
in those cases. (let's hope that different legislative bodies don't
produce conflicting definitions of the same keywords)
The only reason for this is to satisfy the CAN-SPAM Act or any other
national law that mandates topic identification.
I'm not disagreeing with you, just making a statement about what form I
think such laws would take (at least in the US).
I suspect lawmakers would be more sympathetic to the need for a NO
SOLICITATION sign to keep people from being disturbed, than for one to keep
computers from being disturbed. The actual recipients of the message can
avoid being disturbed by filtering mail, if the mail is required to be
labelled. The SMTP extension is just a way to do that more efficiently.
In CAN-SPAM, Section 10. STUDY OF EFFECTS OF COMMERCIAL ELECTRONIC MAIL.
Item #2 says:
I believe this is from an earlier draft. Go reread the law as it was
actually passed by Congress. I don't see IETF mentioned in that law.
Basically as I read the law, IETF isn't in the loop, and there's no
expectation that any protocol extensions are needed.