Haraka

Haraka is an open-source SMTP server written in Node.js. It functions as a Mail Transfer Agent (MTA) or Mail Submission Agent (MSA) designed for high throughput and low latency. The software is deployed as a server application and handles thousands of concurrent connections to deliver messages per second. It is optimized for environments that require the ability to process a high volume of email traffic with minimal delay.

Typical use cases include deploying the server as a filtering MTA or using it on port 465 and legacy port 587 for mail submission with authentication. It includes a built-in outbound delivery engine that automatically queues mail flagged as relaying for delivery. Haraka does not function as a mail store, Local Delivery Agent (LDA), or IMAP server, but instead works alongside those systems to handle the transport and filtering of messages.

Key features

• Modular plugin architecture based on asynchronous JavaScript
• Support for DKIM and SPF verification
• Integration with SpamAssassin, rspamd, and ClamAV
• Connection hooks for connect, helo, mail, rcpt, and data
• Support for DNSBL lookups and Redis integration
• Built-in outbound delivery queue for relaying mail
• Ability to rewrite recipient addresses via custom plugins
• Support for SMTP authentication

The architecture relies on a sequence of well-defined hooks that allow developers to extend server behavior with JavaScript. Because plugins are asynchronous, lookups against HTTP APIs, DNS, or databases do not block the main server process. This design allows administrators to implement complex behaviors, such as accepting tagged addresses and rewriting them before forwarding to an Exchange or IMAP backend, using small JavaScript files. It integrates with various queue backends and external security tools to provide comprehensive spam protection and mail validation.

Haraka is positioned as a scalable infrastructure tool for managing high-volume email traffic and custom SMTP routing.