[phire]

You could deal with large regular mailing lists, you want to move to a system where users can subscribe to a signed mailing list and have the mail server remember that subscription. But yeah, irregular mailing lists where you suddenly need to tell your million users that your database has been breached and they need to change their passwords will be an issue.

But we are talking about $0.0005 (0.05 cents) worth of hashing on CPU systems per email. With GPU based hashing it should be 50 times cheaper. If your team has 1 million customers, and can't afford the $10 to pay a GPU based hashing service to hash 1 million emails for you, you might have a bigger problem.

I suspect the biggest issue for such a scheme is $10 for 1 million emails is potentially still profitable for spammers while being at the limit of how much cpu time you can expect to waste for regular (particularly mobile) users. And that's before you consider the possibility of ASIC hashing for emails.