CURE: Consent Upon Recipient Engagement

This document specifies CURE (Consent Upon Recipient Engagement), a fully native, asynchronous mechanism for expressing and enforcing recipient consent in email without introducing mailbox enumeration, synchronous negotiation, or new SMTP commands. CURE does not guarantee delivery, consent, or sender legitimacy; rather, it provides a privacy-preserving signal that mailbox providers MAY use to improve enforcement and sender accountability.