GuardMail Protocol (GMP): Authenticated Messaging with Non-Repudiable Cryptographic Receipts

This document specifies the GuardMail Protocol (GMP), a framework for authenticated outbound messaging that provides cryptographic proof of origin, integrity, and policy compliance for email and file-based communications. GMP introduces GuardMail Receipts (GMRs), signed metadata structures bound to sender identity and message content that allow recipients to verify authenticity independent of transport mechanisms such as SPF, DKIM, and DMARC. GMP provides a modern, interoperable method for organizations to protect against impersonation, deepfake-based fraud, unauthorized content changes, and spoofed internal communications in enterprise environments.