MailCue

MailCue

MailCue is an open-source (MIT) email testing server that packs a complete mail stack into a single Docker container. Instead of mocking email, it runs real infrastructure — Postfix for SMTP, Dovecot for IMAP/POP3, OpenDKIM and OpenDMARC for authentication, and SpamAssassin for filtering — so messages behave exactly as they would in production.

What's inside

• Full mail stack: catch-all SMTP, IMAP/POP3 with STARTTLS and implicit TLS, DKIM signing, DMARC verification, SPF checks, and spam scoring.
• REST API & SDKs: an OpenAPI 3.1 JSON API with official Python (PyPI) and Node (npm) SDKs, built for CI/CD pipelines.
• MCP server: an official Model Context Protocol server gives AI agents their own mailbox to read, search, send, and triage email.
• Modern web UI: a React interface with mailbox sidebar, folder navigation, rich email viewer, and HTML compose editor.
• Real-time events: Server-Sent Events stream pushes email.received, email.deleted, mailbox.created, and more.
• Multi-user & scoped keys: isolated mailboxes with quotas and X-API-Key permissions limited to specific actions and mailboxes.
• Domain management: custom domains with automatic DKIM plus a DNS dashboard verifying MX, SPF, DKIM, DMARC, MTA-STS, and TLS-RPT.
• Production mode: MAILCUE_MODE=production enables strict domains, enforced TLS, DMARC reject, and Let's Encrypt certificates.
• Extras: GPG/PGP-MIME support, a provider sandbox for SMS/voice/chat APIs, an HTTP request bin, and SQLite or PostgreSQL storage.

Getting started

docker run -d -p 8088:80 -p 25:25 ghcr.io/olib-ai/mailcue

Then check the API health endpoint at http://localhost:8088/api/v1/health. No signup or configuration is required to start testing.

Who it's for

Developers writing end-to-end email tests, teams running CI/CD pipelines, and AI agent builders who need a safe, self-contained mailbox environment. Because it can also run as a hardened production mail server, the same tool covers both testing and real deployment.