So Hashcash was, as far as I'm aware, the first PoW system ever developed, but I'm not aware of it ever actually being deployed as an antispam measure. And indeed, the history of bitcoin also demonstrates why Hashcash would have ultimately failed as a spam-prevention measure: bitcoin can only be effectively mined by large, dedicated farms (or just outright stealing others' resources). There is no clearing price for compute that would have let regular people (especially those on anemic hardware, think "feature phone in Africa") send email while prohibiting people with access to large resources (e.g., botfarms) from mass email.