This is completely absurd - anti-spam PoW is not remotely comparable to crypto miners, and the electricity cost will be so far below the noise floor of owning a computer in the first place that you will literally not notice (and neither will the environment), unless website owners are completely insane and set up multi-second challenges (which, they won't).

And, it's absolutely worth the cost - as a website owner, you get to impose costs on botting operations with minimal penalties for normal users and minimal environmental impact. Bots work because the costs of renting an AWS server and scraping websites (or sending spam, whatever) are extremely tiny - adding PoW challenges to everything that could be spammed suddenly massively changes the cost of running those spam operations, and would result in noticeably less spam if deployed widely.

In fact, the net "environmental impact" would be negative, as botters start to shut down operations due to greatly increased operational costs.

You do PoW every time you send an email.