Comment by gabipurcaru

> The "spammer" can only pretend to be 10 million different people because e-mail is free. Paying a tenth of a penny per e-mail has been one of those long-standing impossible anti-spam measures, but walled gardens can implement something like this at their whim.

Maybe. A few problems here:

1. payments come with privacy concerns, unless maybe you're talking about zero-knowledge-based blockchains, but we're a LONG way from such functionality being widespread

2. $0.001/email is actually very reasonable for an attacker; they'd probably gladly pay even up to $1 or more, depending on their exact needs, especially if that comes with an elevated privileges account

3. all of this is easily defeated by fanouts. E.g. if they sign up with bob@gmail.com and then are able to use bob+1@gmail.com, bob+2@gmail.com etc. to sign up for a different service, this defeats the purpose