A Deployment Profile for DKIM2 via Milter Interface

This document defines a deployment profile for DomainKeys Identified Mail v2 (DKIM2) that is implementable via the existing milter interface without modifications to Mail Transfer Agent (MTA) core software. It identifies a mandatory core profile (DKIM2-core) covering envelope binding, chain of custody, header accountability, replay prevention and DSN authentication and an optional extended profile (DKIM2-extended) covering body recipes and Message-Instance headers. The separation is motivated by deployment realism: the core profile addresses the primary threat models identified in the DKIM2 motivation document and is deployable incrementally across heterogeneous infrastructure, including small operators, universities and research institutions, using the same milter-based deployment model that has proven effective for DKIM1 and ARC. The intent of this document is not to obstruct DKIM2 but to make it deployable. DKIM2-core can be deployed incrementally across the heterogeneous ecosystem in a short timeframe. DKIM2-extended requires significantly longer implementation cycles and may not be deployable in jurisdictions with stricter privacy requirements. Both profiles are part of DKIM2 - the separation serves adoption, not opposition.