Building and scaling software products like Owl Browser taught our team a disappointing lesson: testing transactional email workflows is a big headache. When you need to verify things like DKIM signatures, DMARC alignment, spam filtering, or automatic IMAP parsing, simple SMTP catchers don’t cut it. We were forced to constantly test in an environment that looked nothing like production, which led to unexpected delivery problems.
So, we created mailqueue.
MailCue is a realistic, all-in-one email testing server packaged in a single Docker container. We combined Postfix, Dovecot, OpenDKIM, OpenDMARC, and SpamAssassin and wrapped them in a modern React UI and a FastAPI REST backend.
Here’s what makes it special:
- Realistic Production Behavior: It doesn’t just catch emails; It generates realistic headers, multi-hop receive chains, simulated DKIM-signatures and authentication-results.
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Full API Control: Bypass SMTP completely and inject emails directly through our REST API. Perfect for deterministic testing in your CI/CD pipelines.
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A Complete Mail Stack: Read captured emails via standard IMAP/POP3 clients, or use the built-in responsive web interface.
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Zero Infrastructure Mess: No external database, Redis, or queues required. Everything runs in a container managed by s6-overlay.
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Production ready switches: Set MAILCUE_MODE=PRODUCTION to flip from a catch-all test sandbox to a fully hardened, live email server with strict virtual domains and TLS enforcement.
MailCue is completely open-source (MIT), and we designed it as a shared development dependency that you can easily plug into your Docker network.
We’d love for you to give it a whirl, try to break it down, and let us know what you think. I’ll be hanging out in the comments all day long to answer any questions about architecture, setup, or our roadmap! 🚀
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