MTA Hooks: An HTTP-Based Mail Processing Protocol

This document specifies MTA Hooks, an HTTP-based protocol enabling Mail Transfer Agents (MTAs) to delegate message processing decisions to external services. MTA Hooks provides a modern alternative to legacy mail filtering protocols by leveraging ubiquitous HTTP infrastructure, supporting both JSON and CBOR serialization, and offering fine-grained capability negotiation. The protocol supports both inbound message reception and outbound message delivery scenarios, allowing external scanners to inspect messages, modify content, and influence routing decisions at various stages of mail processing.