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I believe it uses a header the sender includes with the email

Cool. I doubt those intrusive marketing emails will ever include that header though.

it's a neat feature for legit websites not wanting to end up on spam filters, though

Also a cool way for a spammer to verify if somebody is receiving those mails.

True, but that's a low- to no-value lede; you're not very likely to sell to someone that's actively trying to avoid you.

Who cares if they are receiving them and never seeing them because they are blocked?

Someone who easily can send other mails to the now verified address from somewhere else?