Mox

Mox is a full‑stack, self‑hosted email platform written entirely in Go. From the server side it implements SMTP, IMAP4, Webmail, and a REST‑style HTTP API for transactional mail. The application is designed to run as a single binary on a dedicated VM or container, with no external dependencies beyond the operating system and DNS resolver. Its core is a high‑performance mail transport that supports SPF, DKIM, DMARC, DANE, MTA‑STS, and automatic ACME/TLS provisioning, making it suitable for production deployments that require end‑to‑end encryption and strict authentication.

Architecture & Technical Stack

• Language & Runtime: Go 1.22+; the entire codebase follows RFC‑centric design, with extensive automated tests and fuzzing.
• Mail Transport: A custom SMTP/IMAP implementation that handles extensions (STARTTLS, AUTH mechanisms) and implements reputation‑based throttling and Bayesian spam filtering per user.
• Storage: A lightweight, file‑system‑based mailbox format (similar to Maildir) stored under each user’s home directory; configuration and state are kept in plain text YAML files for transparency.
• Web Interface & API: A single integrated web server (port 443) that serves static assets, reverse‑proxy capabilities for hosting websites, and a JSON/HTTP API (/api/v1/...) for sending mail, retrieving events, and webhook integration.
• Metrics & Logging: Prometheus metrics are exposed on /metrics, and structured logs (JSON) can be shipped to any log aggregator.

Core Capabilities for Developers

• Programmable API: Endpoints to send transactional mail, fetch delivery reports, and receive incoming messages via webhooks.
• Extensible Reputation Engine: Developers can hook into the per‑user reputation system to inject custom scoring logic or external blacklists.
• Pluggable Filters: The spam filtering pipeline is modular; custom filters can be added as Go packages and reused across projects.
• Account Autodiscovery: SRV, Microsoft‑style, Thunderbird‑style, and Apple device profiles are auto‑generated, simplifying client configuration.

Deployment & Infrastructure

Mox is container‑ready; a Dockerfile and OCI image are available, making it trivial to spin up in Kubernetes or any container runtime. The binary requires only a user account (mox) and a home directory; it creates its own configuration during the quickstart phase. For high availability, multiple instances can be run behind a load balancer with shared storage (e.g., NFS) for mailboxes, or each instance can serve a distinct domain. The built‑in ACME client means TLS certificates are renewed automatically, eliminating operational overhead.

Integration & Extensibility

The application exposes a webhooks system that can forward delivery status and inbound message events to external services. Its modular design means the core mail processing logic can be imported into other Go projects; many of its non‑server packages are intentionally reusable. The configuration API and web UI allow runtime changes without restarting, which is invaluable for rapid iteration in development environments.

Developer Experience

• Zero External Dependencies: No need to install Postfix, Dovecot, or other mail daemons; everything is bundled.
• Clear Documentation: The source includes RFC‑referenced comments and a comprehensive README; the website hosts a detailed feature list and roadmap.
• Active Community: MIT licensing encourages contribution, and the project’s GitHub repository is actively maintained with frequent releases.
• Testing: Extensive unit, integration, and fuzz tests provide confidence that changes do not regress core protocols.

Use Cases

1. Private SaaS Email Service – A startup can deploy Mox to provide secure, self‑hosted email for its users, leveraging the API for transactional mail while keeping full control over data.
2. Enterprise In‑House Mail Server – Organizations that require strict compliance can run Mox on their own infrastructure, benefiting from built‑in DMARC and DANE support.
3. Developer Sandbox – The mox localserve subcommand allows developers to test email flows locally without hitting external SMTP providers.
4. Hybrid Cloud – Mox can act as an edge server that forwards mail to a cloud provider, combining local control with external deliverability.

Advantages Over Alternatives

• Performance & Simplicity: A single Go binary eliminates the overhead of multiple daemons; the code is lean and well‑tested.
• Security by Design: Automatic TLS, strict authentication checks, and reputation‑based throttling reduce the attack surface.
• Licensing & Freedom: MIT license permits commercial use without copyleft constraints, unlike some open‑source mail servers.
• Developer‑Friendly Extensibility: Reusable Go packages and a programmable API make it easier to integrate Mox into existing tooling compared to more monolithic solutions.

Mox thus offers developers a complete, secure, and highly extensible email stack that can be deployed with minimal operational burden while still exposing powerful hooks for customization and automation.