We needed a stamp.

Regulated "Emails cost 1 penny" would have worked fine. All you need to do to meaningfully fight spam is have a cost that isn't completely negligible; Spammers started out at a rate where they spend less than a day's wages to message literally every human being on the planet; At those costs even finding a single person you can convince of your Nigerian prince account nets you a profit.

We controlled the pipes and the formats in the 90's and 00's almost unilaterally. We should have made a stamp.