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Because then all the robots (vacation responders) reply too. The separate address in the body of the email is a little like a captcha humanity test.

Reply to those autoreplies. Crash the mailboxes of people using broken autoresponders.

Easy solution: Mandate that auto responses are marked as such in a header.

Obvious problem: the same impossibility if enforcing this applies to this solution as well.

Neither is impossible to enforce.

Please do explain how.

You want me to explain the concept of law enforcement? Or of fines? You're the one making a strong claim here.