Overview

MTA Hooks is a modern replacement for the milter protocol, designed to provide enhanced flexibility and ease of use for managing and processing email transactions within Mail Transfer Agents (MTAs). Unlike milter, which operates at a lower level and uses a custom protocol, MTA Hooks leverages the ubiquitous HTTP protocol, making it simpler to integrate and deploy.

MTA Hooks is built on the robust and widely used HTTP protocol, making integration and debugging significantly simpler compared to traditional methods. The protocol uses JSON for both requests and responses, offering a clear, human-readable format that enhances the ease of implementation. Each stage of the SMTP transaction is managed by sending a POST request to a specified HTTP endpoint, ensuring seamless communication and processing. MTA Hooks can be invoked at any point in the SMTP transaction, from the initial connection phase to the final message delivery, providing comprehensive coverage and control over the email processing workflow.

The typical workflow of MTA Hooks begins when the Mail Transfer Agent (MTA) reaches a particular stage in the SMTP transaction. At this point, the MTA sends a POST request containing a JSON payload to the MTA Hooks endpoint. This payload includes detailed information about the transaction, such as client and server details, message envelope, and message content. The endpoint processes this request and responds with a JSON object specifying the action to be taken by the MTA. This response can include directives such as accepting, rejecting, discarding, or quarantining the message, along with any necessary modifications to the message or transaction. Based on the response received, the MTA executes the specified actions, applying any modifications as instructed, thus completing the transaction stage.