Posthorn

https://posthorn.dev
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Posthorn is a self-hosted mail gateway that acts as an outbound layer between applications and...
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### Install Posthorn from Source

Source: https://posthorn.dev/deployment/binary

Install the Posthorn binary using Go modules. Requires Go 1.25+.

```bash
go install github.com/craigmccaskill/posthorn/cmd/posthorn@latest
```

--------------------------------

### Install Posthorn from Release Artifacts

Source: https://posthorn.dev/deployment/binary

Download, extract, and install the pre-built binary from GitHub releases.

```bash
curl -L -o posthorn.tar.gz \
  https://github.com/craigmccaskill/posthorn/releases/download/v1.0.0/posthorn-linux-amd64.tar.gz
tar -xzf posthorn.tar.gz
sudo install -m 0755 posthorn /usr/local/bin/posthorn
```

--------------------------------

### Serve Command with Config

Source: https://posthorn.dev/reference/cli

Starts the posthorn listener using a specified configuration file.

```bash
posthorn serve --config /etc/posthorn/config.toml
```

--------------------------------

### Verify Posthorn Installation with Standalone Binary

Source: https://posthorn.dev/getting-started/installation

Run this command to verify the Posthorn installation when using the standalone binary. It checks the version of the installed tool.

```bash
posthorn version
```

--------------------------------

### Curl Examples for HTTP POST Requests

Source: https://posthorn.dev/reference/http-api

Demonstrates how to make POST requests to the API using curl. Includes examples for standard form data, multipart form data, and forcing a JSON response by setting the `Accept` header.

```bash
# Success
curl -X POST http://localhost:8080/api/contact \
  -H "Content-Type: application/x-www-form-urlencoded" \
  -d "name=Test User&email=test@example.com&message=hello"

```

```bash
# Multipart
curl -X POST http://localhost:8080/api/contact \
  -F name="Test User" -F email=test@example.com -F message=hello

```

```bash
# Force JSON response
curl -X POST http://localhost:8080/api/contact \
  -H "Accept: application/json" \
  -d "name=...&..."

```

--------------------------------

### Start and Enable Posthorn Service

Source: https://posthorn.dev/deployment/binary

Manage the Posthorn systemd service, including starting, enabling, and checking status.

```bash
sudo systemctl daemon-reload
sudo systemctl enable --now posthorn
sudo systemctl status posthorn
```

--------------------------------

### Template Rendering Example

Source: https://posthorn.dev/features/api-mode

Demonstrates how keys from a flat JSON body are rendered as template variables.

```go-template
Name: {{.name}}            → Alice
Count: {{.count}}          → 42
Confirmed: {{.confirmed}}  → true
Tags: {{.tags}}            → [urgent support]
```

--------------------------------

### Run Posthorn CLI Commands

Source: https://posthorn.dev/deployment/binary

Examples of using the 'validate' and 'serve' CLI commands.

```bash
posthorn validate --config /etc/posthorn/config.toml
posthorn serve --config /etc/posthorn/config.toml
```

--------------------------------

### Successful Posthorn Startup Logs

Source: https://posthorn.dev/reference/cli

Example of structured JSON logs indicating a successful posthorn service startup.

```json
{"time":"2026-05-16T19:00:00Z","level":"INFO","msg":"posthorn starting","version":"v1.0.0","listen":":8080","config":"/etc/posthorn/config.toml","endpoints":2}
{"time":"2026-05-16T19:00:00Z","level":"INFO","msg":"endpoint registered","path":"/api/contact","transport":"postmark","recipients":1}
{"time":"2026-05-16T19:00:00Z","level":"INFO","msg":"http ingress listening","addr":":8080"}
```

--------------------------------

### Verify Posthorn Installation

Source: https://posthorn.dev/deployment/binary

Check the installed Posthorn version.

```bash
posthorn --version
# posthorn v1.0.0
```

--------------------------------

### API Mode Request Example

Source: https://posthorn.dev/reference/http-api

A minimal example demonstrating a POST request to the API endpoint with Bearer authentication, JSON body, and idempotency key.

```http
POST /api/transactional HTTP/1.1
Authorization: Bearer abc123
Content-Type: application/json
Idempotency-Key: reset:user-42:2026-05-16T20


{
  "to_override": "alice@example.com",
  "subject_line": "Reset your password",
  "message": "https://app.example.com/reset/xyz"
}
```

--------------------------------

### Verify Posthorn Installation with Docker

Source: https://posthorn.dev/getting-started/installation

Run this command to verify the Posthorn installation when using Docker. It checks the version of the running container.

```bash
docker run --rm ghcr.io/craigmccaskill/posthorn:latest version
# posthorn v1.0.0
```

--------------------------------

### PostHog Process Start Event

Source: https://posthorn.dev/reference/log-format

Emitted once when the PostHog process starts. Includes version, listening address, configuration path, and number of endpoints.

```json
{
  "level": "INFO",
  "msg": "posthorn starting",
  "version": "v1.0.0",
  "listen": ":8080",
  "config": "/etc/posthorn/config.toml",
  "endpoints": 2
}
```

--------------------------------

### Run Posthorn Commands

Source: https://posthorn.dev/getting-started/installation

Commands to validate the configuration file and start the Posthorn HTTP listener using the standalone binary.

```bash
posthorn validate --config posthorn.toml   # check the config without serving
posthorn serve --config posthorn.toml      # start the HTTP listener
```

--------------------------------

### Complete PostHog Configuration Example

Source: https://posthorn.dev/configuration/reference

This example demonstrates a full PostHog configuration, including two HTTP endpoints for a contact form and a transactional API, along with an SMTP listener. It covers logging, endpoint-specific settings like 'to', 'from', 'required' fields, email templating, transport configurations (Postmark, Resend), rate limiting, and SMTP listener settings such as TLS, authentication, and allowed senders.

```toml
# Two HTTP endpoints (a contact form + a transactional api-mode endpoint)
# plus an SMTP listener for Ghost/Gitea/etc.


[logging]
level = "info"
format = "json"


[[endpoints]]
path = "/api/contact"
to = ["alerts@example.com"]
from = "Contact Form <noreply@example.com>"
required = ["name", "email", "message"]
email_field = "email"
honeypot = "_gotcha"
allowed_origins = ["https://example.com", "https://www.example.com"]
max_body_size = "64KB"
trusted_proxies = ["cloudflare"]
csrf_secret = "${env.CSRF_SECRET}"
csrf_token_ttl = "30m"
subject = "Contact: {{.name}}"
body = """
From: {{.name}} <{{.email}}>


{{.message}}
"""
redirect_success = "/thank-you"
redirect_error = "/contact?error=1"


[endpoints.transport]
type = "postmark"


[endpoints.transport.settings]
api_key = "${env.POSTMARK_CONTACT_KEY}"


[endpoints.rate_limit]
count = 5
interval = "1m"

# ---


[[endpoints]]
path = "/api/transactional"
to = ["fallback@example.com"]
from = "App <noreply@example.com>"
auth = "api-key"
api_keys = ["${env.WORKER_KEY_PRIMARY}", "${env.WORKER_KEY_BACKUP}"]
idempotency_cache_size = 5000
required = ["subject_line", "body"]
subject = "{{.subject_line}}"
body = "{{.body}}"


[endpoints.transport]
type = "resend"


[endpoints.transport.settings]
api_key = "${env.RESEND_API_KEY}"


[endpoints.rate_limit]
count = 100
interval = "1m"


# ---


[smtp_listener]
listen = ":2525"
require_tls = true
tls_cert = "/etc/posthorn/cert.pem"
tls_key = "/etc/posthorn/key.pem"
auth_required = "smtp-auth"
allowed_senders = ["*@example.com"]
max_recipients_per_session = 10
max_message_size = "1MB"


[[smtp_listener.smtp_users]]
username = "ghost"
password = "${env.GHOST_SMTP_PASSWORD}"


[smtp_listener.transport]
type = "postmark"


[smtp_listener.transport.settings]
api_key = "${env.POSTMARK_API_KEY}"

```

--------------------------------

### Start and Monitor Posthorn Container

Source: https://posthorn.dev/getting-started/quick-start

Start the Posthorn container in detached mode and follow its logs to monitor its status and activity.

```bash
docker compose up -d
docker compose logs -f posthorn
```

--------------------------------

### API Mode Response Example

Source: https://posthorn.dev/reference/http-api

An example of a successful HTTP 200 OK response from the API, indicating the submission status and ID.

```http
HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8


{"status":"ok","submission_id":"..."}
```

--------------------------------

### Posthorn Service Startup Logs

Source: https://posthorn.dev/recipes/multi-form-site

Example log output from Posthorn upon successful startup, indicating that endpoints have been registered and the HTTP ingress is listening.

```log
{"time":"...","level":"INFO","msg":"endpoint registered","path":"/api/contact","transport":"postmark","recipients":1}
{"time":"...","level":"INFO","msg":"endpoint registered","path":"/api/feedback","transport":"postmark","recipients":1}
{"time":"...","level":"INFO","msg":"endpoint registered","path":"/api/careers","transport":"postmark","recipients":1}
{"time":"...","level":"INFO","msg":"http ingress listening","addr":":8080"}

```

--------------------------------

### Start Posthorn Container (First Endpoint)

Source: https://posthorn.dev/recipes/newsletter-signup

Command to start the Posthorn Docker container in detached mode if it's the first endpoint being configured.

```bash
docker compose up -d
```

--------------------------------

### Trusted Proxies Configuration

Source: https://posthorn.dev/configuration/reference

Example of configuring trusted proxy IP ranges and presets for client IP handling.

```toml
trusted_proxies = ["cloudflare", "10.0.0.0/8"]
```

--------------------------------

### Set Environment Variables with systemd

Source: https://posthorn.dev/security/api-keys

Example of setting environment variables for Posthorn using systemd's EnvironmentFile directive.

```bash
# systemd
EnvironmentFile=/etc/posthorn/posthorn.env
```

--------------------------------

### Configure a Transport

Source: https://posthorn.dev/getting-started/concepts

Specify the transport type and its settings for an endpoint. This example shows configuration for the Postmark transport.

```toml
[endpoints.transport]
type = "postmark"


[endpoints.transport.settings]
api_key = "${env.POSTMARK_API_KEY}"
```

--------------------------------

### Configure Transport API Key

Source: https://posthorn.dev/security/api-keys

Example of configuring a transport API key using an environment variable placeholder in TOML.

```toml
# Transport credential
[endpoints.transport.settings]
api_key = "${env.POSTMARK_API_KEY}"
```

--------------------------------

### Display Posthorn Version

Source: https://posthorn.dev/reference/cli

Use the `version` command to display the installed version of Posthorn. This command is aliased to `--version` and `-v`.

```bash
$ posthorn version
posthorn v1.0.0

```

--------------------------------

### Set Environment Variables Directly

Source: https://posthorn.dev/security/api-keys

Example of setting environment variables directly when running Posthorn from the command line.

```bash
# direct
POSTMARK_API_KEY=abc123 posthorn serve --config ...
```

--------------------------------

### Example of Go text/template Syntax

Source: https://posthorn.dev/features/templating

Illustrates common Go text/template syntax elements like variables, pipes, conditionals, and comments. These are used for rendering dynamic content in email subjects and bodies.

```go-template
{{.name}}
```

```go-template
{{.email | html}}
```

```go-template
{{if .urgent}}!!!{{end}}
```

```go-template
{{/* this is a comment */}}
```

```go-template
{{if eq .category "bug"}}
```

--------------------------------

### jq Queries for Docker Logs

Source: https://posthorn.dev/reference/log-format

Examples of using jq to parse and filter Docker logs for the 'posthorn' container. These snippets demonstrate ad-hoc analysis for errors, latency, and specific events.

```bash
# Tail and pretty-print
docker logs -f posthorn | jq .
```

```bash
# All failures from the last hour
docker logs --since 1h posthorn | jq 'select(.level=="ERROR")'
```

```bash
# Latency P99 over the last 1000 sent submissions
docker logs --tail 10000 posthorn \
  | jq -s 'map(select(.msg=="submission_sent")) | sort_by(.latency_ms) | .[-10:]'
```

```bash
# Find a specific submission across all log lines
docker logs posthorn | jq 'select(.submission_id=="7f2c84d6-9b1e-4c2f-a3b8-1a2b3c4d5e6f")'
```

```bash
# Find a specific SMTP session across all log lines
docker logs posthorn | jq 'select(.session_id=="...")'
```

--------------------------------

### Configure log rotation for standalone Posthorn logs

Source: https://posthorn.dev/deployment/reading-logs

Example `logrotate` configuration for managing log files generated by a standalone Posthorn binary.

```logrotate
/var/log/posthorn.log {
    daily
    rotate 14
    compress
    missingok
    notifempty
    copytruncate
}
```

--------------------------------

### TOML Configuration with API Key Placeholder

Source: https://posthorn.dev/configuration/environment-variables

Use `${env.VAR}` syntax to inject environment variables into TOML string fields. This example shows how to configure an API key.

```toml
api_key = "${env.POSTMARK_API_KEY}"
```

--------------------------------

### Configure API-mode Bearer Tokens

Source: https://posthorn.dev/security/api-keys

Example of configuring API-mode bearer tokens using environment variable placeholders in TOML.

```toml
# API-mode bearer tokens
[[endpoints]]
auth = "api-key"
api_keys = ["${env.WORKER_KEY_PRIMARY}", "${env.WORKER_KEY_BACKUP}"]
```

--------------------------------

### Set Environment Variables with Docker Compose

Source: https://posthorn.dev/security/api-keys

Example of setting environment variables for Posthorn using a Docker Compose file.

```bash
# Docker Compose
docker compose --env-file .env up -d
```

--------------------------------

### Configure Reply-To Email and Body Template

Source: https://posthorn.dev/features/validation

Example TOML configuration for setting required fields, the reply-to email field, and a body template that includes the sender's name and email.

```toml
[[endpoints]]
required = ["name", "email", "message"]
reply_to_email_field = "email"
subject = "Contact from {{.name}}"
body = """
From: {{.name}} <{{.email}}>


{{.message}}
"""
```

--------------------------------

### Configure SMTP User Password

Source: https://posthorn.dev/security/api-keys

Example of configuring an SMTP user password using an environment variable placeholder in TOML.

```toml
# SMTP user password
[[smtp_listener.smtp_users]]
username = "ghost"
password = "${env.GHOST_SMTP_PASSWORD}"
```

--------------------------------

### Configure CSRF HMAC Key

Source: https://posthorn.dev/security/api-keys

Example of configuring a CSRF HMAC key using an environment variable placeholder in TOML.

```toml
# CSRF HMAC key
csrf_secret = "${env.CSRF_SECRET}"
```

--------------------------------

### Successful Validation Output

Source: https://posthorn.dev/reference/cli

Displays a success message when the configuration is valid and the listener is ready to start. Exit code 0 indicates success.

```bash
$ posthorn validate --config posthorn.toml
config OK: 2 endpoint(s)
```

--------------------------------

### Multipart POST Request Example

Source: https://posthorn.dev/reference/http-api

This snippet demonstrates a POST request using `multipart/form-data` for a form-mode endpoint. It illustrates how form fields are structured within the multipart body, separated by a boundary.

```http
POST /api/contact HTTP/1.1
Host: example.com
Content-Type: multipart/form-data; boundary=----WebKitFormBoundary

------WebKitFormBoundary
Content-Disposition: form-data; name="name"

Alice Smith
------WebKitFormBoundary
Content-Disposition: form-data; name="email"

alice@example.com
------WebKitFormBoundary
Content-Disposition: form-data; name="message"

Hello world
------WebKitFormBoundary--
```

--------------------------------

### Posthorn Service Logs

Source: https://posthorn.dev/recipes/hugo-comentario

Example log output from Posthorn indicating successful registration of HTTP and SMTP listeners, and endpoint registration for Postmark.

```json
{"msg":"endpoint registered","path":"/api/contact","transport":"postmark","recipients":1}
{"msg":"smtp_listener registered","listen":":2525","transport":"postmark","smtp_users":0}
{"msg":"http ingress listening","addr":":8080"}
{"msg":"smtp ingress listening","addr":":2525"}
```

--------------------------------

### Example Idempotency-Key Request

Source: https://posthorn.dev/features/api-mode

This cURL command demonstrates how to send a POST request with the `Idempotency-Key` header to the transactional API. Ensure the key is a printable ASCII string.

```bash
curl -X POST https://yourdomain.com/api/transactional \
  -H "Authorization: Bearer $WORKER_KEY_PRIMARY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: ord-12345-welcome-email" \
  --data '{"subject_line":"Welcome","message":"..."}'
```

--------------------------------

### Subject Template Parse Error Example

Source: https://posthorn.dev/features/templating

Demonstrates a typical error message when a subject template fails to parse during configuration validation. Syntax errors are caught at deploy time.

```bash
$ posthorn validate --config posthorn.toml
ERROR: endpoint 0 (/api/contact): subject template parse error at line 1: function "foo" not defined
exit status 1

```

--------------------------------

### Build Posthorn from Repository

Source: https://posthorn.dev/deployment/binary

Clone the Posthorn repository and build the binary locally.

```bash
git clone https://github.com/craigmccaskill/posthorn
cd posthorn
go build -o posthorn ./core/cmd/posthorn
```

--------------------------------

### Create Project Directory

Source: https://posthorn.dev/getting-started/quick-start

Create a new directory for your Posthorn project and navigate into it.

```bash
mkdir posthorn-test && cd posthorn-test
```

--------------------------------

### Successful Submission Response

Source: https://posthorn.dev/recipes/contact-form

Example of a successful HTTP response from Posthorn after a valid form submission.

```http
HTTP/2 200
content-type: application/json; charset=utf-8


{"status":"ok","submission_id":"7f2c84d6-9b1e-4c2f-a3b8-1a2b3c4d5e6f"}
```

--------------------------------

### Ghost SMTP Configuration Example

Source: https://posthorn.dev/features/smtp-ingress

Shows environment variables or config keys for configuring Ghost to use Posthorn's SMTP listener. Ensure mail__from matches Posthorn's allowed_senders and auth credentials align with smtp_listener.smtp_users.

```env
mail__from='"Ghost" <noreply@yourdomain.com>'
mail__transport=SMTP
mail__options__host=posthorn.yourdomain.com
mail__options__port=2525
mail__options__secure=false          # using STARTTLS, not implicit TLS
mail__options__auth__user=ghost
mail__options__auth__pass="$GHOST_SMTP_PASSWORD"

```

--------------------------------

### Create Posthorn Directory

Source: https://posthorn.dev/recipes/contact-form

Initializes the directory structure for the Posthorn deployment.

```bash
mkdir -p posthorn && cd posthorn
```

--------------------------------

### Display Posthorn Help Information

Source: https://posthorn.dev/reference/cli

Use the `help` command to display general usage information for the Posthorn CLI. Subcommand-specific help is not separately available; the top-level help lists flags for each subcommand.

```bash
$ posthorn help
posthorn — outbound mail gateway


Usage:
  posthorn serve     [--config <path>] [--listen <addr>]
  posthorn validate  [--config <path>]
  posthorn version
  posthorn help


Examples:
  posthorn serve --config /etc/posthorn/config.toml
  posthorn validate --config ./posthorn.toml


```

--------------------------------

### Add SPF Record (TXT)

Source: https://posthorn.dev/security/dns

Configure a TXT record for SPF. If an SPF record already exists, merge the Postmark include with existing ones instead of creating a duplicate.

```dns
Type:  TXT
Host:  @            (or example.com — depends on DNS provider)
Value: v=spf1 a mx include:spf.mtasv.net ~all
```

```dns
v=spf1 a mx include:_spf.google.com include:spf.mtasv.net ~all
```

--------------------------------

### Caddy Reverse Proxy Configuration

Source: https://posthorn.dev/getting-started/quick-start

Configure Caddy to proxy requests to the Posthorn API. This setup is suitable for static sites.

```caddy
example.com {
    reverse_proxy /api/contact* posthorn:8080
    file_server
}
```

--------------------------------

### Honeypot HTML Input Field

Source: https://posthorn.dev/features/spam-protection

Example of an HTML input field designed to be hidden from users but visible to bots, acting as a honeypot.

```html
<input type="text" name="_gotcha" tabindex="-1" autocomplete="off"
       style="position:absolute;left:-9999px">
```

--------------------------------

### Add DMARC Record (TXT) - Initial Monitoring

Source: https://posthorn.dev/security/dns

Configure a TXT record for DMARC with 'p=none' for monitoring without enforcement. This is recommended before moving to stricter policies.

```dns
v=DMARC1; p=none; rua=mailto:dmarc-reports@example.com
```

--------------------------------

### Cron Job for Simple Alerting

Source: https://posthorn.dev/recipes/monitoring-alerts

A cron job example that checks a service's health and sends a DOWN alert via Posthorn if it fails.

```bash
*/5 * * * * curl -s --fail https://api.yourservice.com/healthz || \
  curl -s -X POST https://yourdomain.com/api/alert \
    --data-urlencode "service=api.yourservice.com" \
    --data-urlencode "status=DOWN" \
    --data-urlencode "details=healthz check failed at $(date -Iseconds)"
```

--------------------------------

### Environment Variable Configuration for Postmark API Keys

Source: https://posthorn.dev/recipes/multi-form-site

Sets up environment variables for Postmark API keys, supporting both a single token for all endpoints or individual tokens for each.

```dotenv
# One-token shape:
# POSTMARK_API_KEY=your-token


# Or three-token shape:
POSTMARK_CONTACT_KEY=your-contact-server-token
POSTMARK_FEEDBACK_KEY=your-feedback-server-token
POSTMARK_CAREERS_KEY=your-careers-server-token

```

--------------------------------

### Custom Fields Passthrough Example

Source: https://posthorn.dev/getting-started/concepts

Demonstrates how additional form fields beyond the named fields are included in the email body under 'Additional fields'.

```text
From: Alice <alice@example.com>


This is the message body the template rendered.


Additional fields:
  company: Acme Corp
  source: HN
  newsletter_opt_in: yes
```

--------------------------------

### Posthorn Configuration (posthorn.toml)

Source: https://posthorn.dev/recipes/ghost

Sets up Posthorn's SMTP listener to accept mail from Ghost. Configures sender allowlist, message size, and transport settings for Postmark.

```toml
[smtp_listener]
listen          = ":2525"
auth_required   = "none"
require_tls     = false
allowed_senders = ["*@example.com"]
max_recipients_per_session = 50    # higher than default; newsletter sends batch
max_message_size = "10MB"          # bigger than default; newsletters embed images


[smtp_listener.transport]
type = "postmark"


[smtp_listener.transport.settings]
api_key = "${env.POSTMARK_API_KEY}"
```

--------------------------------

### Example Submission ID in JSON Response

Source: https://posthorn.dev/getting-started/concepts

A sample JSON response containing the generated submission ID, used for correlating log events.

```json
{"submission_id":"7f2c84d6-9b1e-4c2f-a3b8-1a2b3c4d5e6f", ...}
```

--------------------------------

### Request Pipeline Stages

Source: https://posthorn.dev/getting-started/concepts

Illustrates the sequential processing steps for form submissions, from body size capping to response logging.

```text
1. body size cap        → http.MaxBytesReader wraps r.Body   (413)
2. method check         → POST only                          (405)
3. content-type check   → form-encoded only                  (400)
4. origin/referer check → fail-closed if allowed_origins set (403)
5. rate limit check     → token bucket, proxy-aware IP       (429)
6. parse form           → r.ParseForm() reads body           (413/400)
7. honeypot check       → silent 200 if field non-empty      (200)
8. required fields      → all listed fields present + non-empty (422)
9. email format         → submitter email field syntactic    (422)
10. generate submission ID (UUIDv4), log "submission_received"
11. render subject template
12. render body template + custom-fields passthrough
13. transport.Send() with retry policy
14. log outcome, write response (JSON or redirect)           (200/502)
```

--------------------------------

### Configuring a Transport Type

Source: https://posthorn.dev/configuration/transports

Example TOML configuration for setting the email transport type for an endpoint. Supports Postmark, Resend, Mailgun, SES, and SMTP.

```toml
[endpoints.transport]
type = "postmark"  # or "resend", "mailgun", "ses", "smtp"


[endpoints.transport.settings]
# Per-transport settings, see the matching page below.
```

--------------------------------

### 303 See Other Redirect

Source: https://posthorn.dev/reference/response-codes

Returned after content negotiation chooses an HTML-friendly response, typically for form submissions. Ensures the browser issues a GET to the redirect URL.

```HTTP
HTTP/1.1 303 See Other
Location: /thank-you

```

--------------------------------

### Add DKIM Record (TXT)

Source: https://posthorn.dev/security/dns

Set up a TXT record for DKIM using the selector and public key provided by Postmark. The selector value will vary.

```dns
Type:  TXT
Host:  20240101pm._domainkey   (Postmark provides the selector — varies)
Value: k=rsa; p=MIGfMA0GCSq...   (long key, Postmark provides full value)
```

--------------------------------

### 500 Internal Server Error Example

Source: https://posthorn.dev/reference/response-codes

This snippet shows the structure of a 500 Internal Server Error response, indicating a Posthorn-internal failure such as a template rendering issue.

```http
HTTP/1.1 500 Internal Server Error
Content-Type: text/plain; charset=utf-8

submission could not be processed
```

--------------------------------

### Ghost Environment Configuration (.env.ghost)

Source: https://posthorn.dev/recipes/ghost

Configures Ghost's mail settings to use Posthorn as the SMTP server. Ensure mail.from matches Posthorn's allowed_senders.

```env
url=https://blog.example.com
database__client=mysql
database__connection__host=ghost-db
database__connection__user=ghost
database__connection__password=${GHOST_DB_PASSWORD}
database__connection__database=ghost


# Mail config. Posthorn is the SMTP server.
mail__transport=SMTP
mail__from="Blog <noreply@example.com>"
mail__options__host=posthorn
mail__options__port=2525
mail__options__secure=false
mail__options__ignoreTLS=true
# mail__options__auth__user and mail__options__auth__pass intentionally omitted.
```

--------------------------------

### Live tail Kubernetes logs

Source: https://posthorn.dev/deployment/reading-logs

Continuously stream logs from a Posthorn deployment in Kubernetes.

```bash
kubectl logs -f deployment/posthorn
```

--------------------------------

### Custom Fields Passthrough Example

Source: https://posthorn.dev/features/templating

Custom fields submitted via a form are automatically appended to the email body in a sorted list. Empty fields are omitted.

```text
[rendered body template]


Additional fields:
  company: Acme Corp
  source: HN
  newsletter_opt_in: yes


```

--------------------------------

### 303 See Other

Source: https://posthorn.dev/reference/response-codes

Indicates a redirect after content negotiation chose an HTML-friendly response. This is typically used after form submissions where the browser should perform a GET request to the specified location.

```APIDOC
## 303 See Other

### Description

Indicates a redirect after content negotiation chose an HTML-friendly response. This is typically used after form submissions where the browser should perform a GET request to the specified location.

### Method

GET (implicitly, by browser behavior)

### Endpoint

N/A (Response to a previous request)

### Response

#### Success Response (303)

- **Location** (string) - The URL to redirect to.

### Response Example

```http
HTTP/1.1 303 See Other
Location: /thank-you
```

### Returned When

* Client prefers `text/html` (e.g., default browser form submission).
* `redirect_success` or `redirect_error` is configured for the endpoint.
* The submission has been processed (successfully or not).

**Note:** The 303 status ensures the browser issues a GET to the redirect URL, preventing form re-submission on refresh.
```

--------------------------------

### Create Unprivileged User for Posthorn

Source: https://posthorn.dev/deployment/binary

Create a system user for running the Posthorn service.

```bash
sudo useradd --system --no-create-home --shell /usr/sbin/nologin posthorn
```

--------------------------------

### HTTP 429 Response Example

Source: https://posthorn.dev/features/rate-limiting

Posthorn returns an HTTP 429 status code with a plaintext body when a rate limit is exceeded. Clients should implement their own backoff strategy.

```http
HTTP/1.1 429 Too Many Requests
Content-Type: text/plain; charset=utf-8


rate limit exceeded

```
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