Isn't spam a ISP issue?

Last couple of days traffic is amazing. A group declared dead, with so much
traffic.

Anyway, not been active since 2004,no longer managing mail servers, the
comments awoke an old question, why is spam the receiver's issue?  It should
be the sending ISP's issue. Yes, some spammers has their own MTAs' sending out
on their own, but of those spams ending up in my spambox, most still seem to
be ISP accounts.

Someone talked about e-stamps; though generally not functional, there is a
sideline of it. If we can get a couple of additions to the SMTP rfcs'. A
standard block of mailing out more than X mails within an hour, anyone wanting
to use a mail list simply has to pay their ISP money to get a escrow key to
the bulk sender interface.

1. The senders get more public, easier to find, either through the ISP
agreement or his own MTA's IP showing up.
2. The cost increases for bulk mailers, and that will stop a lot, the more you
send the costlier.
3. ISP not following rules, allowing bulk mailers outside agreement, they get
blocked, as some suggested.
4. If readily identifiable, spammers can be hit with legal actions or civil
class action suits
5. No change in clients, no more lists to check, same SMTP protocol for 99% of
us.

Drawback, the "legit" bulk sending need to be hard to fake.

Curb spam in the sending end, not the receiving (should be possible also with
spam-SMS), as previous suggested, hit them with increasing cost, but not with
e-stams, but communications fees. Spammers will never be curbed with
technology, only by fees, legal or civil actions.

Just a short reflection of what I read here last hour.
Cheers/Kurt M