Identities are infinite and free.
Sadly, this is somewhat true, but SPF and DKIM work on domain names, and
they typically aren't free. Assuming a domain name seller acquires
payment for the domain, there's is a financial trace possible when
necessary. And if the payment is fraudulent, the domain name seller has
an interest to fix that loophole. While subdomains can be done for
free, they are tied to a domain that again has a reputation to protect
or is traceable if used repeatedly.
To be sure, it ain't easy, and getting worldwide adoption of prepaid
postage is likely harder.
David
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