Posthorn

Posthorn is the unified outbound mail layer for self-hosted projects — one gateway between your apps and your transactional mail provider.

One container. One TOML config. One set of credentials. Apps point at Posthorn, Posthorn points at your provider.

Highlights

• Three ingress shapes — HTTP form (contact forms on static sites), HTTP API with Bearer auth + idempotency (server-to-server callers), and an SMTP listener (Ghost, Gitea, Mastodon, Matrix, NextCloud, Authentik — anything that only speaks SMTP, including on cloud hosts that block outbound 25/465/587).
• Five transports — Postmark, Resend, Mailgun, AWS SES, and outbound-SMTP relay. Provider is a config field; switching is one line and a container restart.
• Bespoke clients, no SDKs — every transport is a 200–300-line stdlib HTTP client. Three external dependencies in the entire Go module: a TOML parser, a UUID library, an LRU cache.
• Security primitives in by default — structured-JSON header construction (submitter input can't become headers), honeypot 200s that are byte-identical to real successes, fail-closed Origin checks, bounded-memory token-bucket rate limiter, per-IP brute-force defense on API endpoints, API keys that are header values only and never logged.
• Operational surface — `/healthz`, `/metrics` (hand-rolled Prometheus exposition, no `prometheus/client_golang` dep), structured JSON logs with UUID submission IDs and `transport_message_id` for jumping to the provider's UI, dry-run mode, CSRF tokens, IP-stripping, named `trusted_proxies` presets.

Get it

```bash
docker pull ghcr.io/craigmccaskill/posthorn:v1.0.0
```

Multi-arch image: `linux/amd64` + `linux/arm64`.

Ten-minute walkthrough from `docker compose up` to a working contact form: posthorn.dev/recipes/contact-form.

What's not in v1.0

No persistent storage — a send that exhausts both retries is gone except for the log line. Durable retry across restarts is on the roadmap. Suppression lists, lifecycle webhooks, HTML body, attachments → all v2.

Links

• Docs: posthorn.dev
• Quick start: posthorn.dev/getting-started/quick-start
• TOML reference: posthorn.dev/configuration/reference
• Design principles: posthorn.dev/design-principles
• Roadmap: posthorn.dev/roadmap
• Full changelog: CHANGELOG.md

Apache-2.0. Bug reports and feature requests: Issues.