That's why I believe that having a stricter form of authentication
would be useful - so the ISP will be able to locate the relevant
sender's user account from the email trace info. Either it could be
that the sender's email address must be permitted by the account
details, or that the authentication information is encoded into the
trace info somehow. (This wouldn't necessarily preclude anonymous
mailing, just 'pseudonymous' mailing)
How do you want to prevent users from accumulating millions of
pseudonyms?
User registration is an extremely tough problem, and nobody has
developed anything that withstands the continuous assaults from spam
gangs and the lunatic fringe.