Migrate from Postmark

Get your Postkit API key

Sign up at app.postkit.eu and create an API key in Settings > API Keys. Production keys start with pk_live_. For testing the migration without sending real emails, create a sandbox key starting with pk_test_.

Update authentication

Replace Postmark's custom X-Postmark-Server-Token header with the standard Authorization: Bearer header.

Before (Postmark):

curl -X POST "https://api.postmarkapp.com/email" \
  -H "X-Postmark-Server-Token: your-server-token" \
  -H "Content-Type: application/json"

After (Postkit):

curl -X POST https://api.postkit.eu/v1/emails \
  -H "Authorization: Bearer pk_live_abc123..." \
  -H "Content-Type: application/json"

Update JSON field names

This is the most impactful change. Postmark uses PascalCase for all JSON fields, while Postkit uses snake_case.

Before (Postmark):

{
  "From": "you@yourdomain.com",
  "To": "user@example.com",
  "Subject": "Hello from Postmark",
  "HtmlBody": "<h1>Hello</h1>",
  "TextBody": "Hello",
  "ReplyTo": "reply@yourdomain.com",
  "Tag": "welcome"
}

After (Postkit):

{
  "from": "you@yourdomain.com",
  "to": ["user@example.com"],
  "subject": "Hello from Postkit",
  "html": "<h1>Hello</h1>",
  "text": "Hello",
  "reply_to": "reply@yourdomain.com",
  "tags": ["welcome"]
}

Common field mappings:

Unify template sending

Postmark uses a separate endpoint for template-based emails (POST /email/withTemplate). Postkit uses the standard send endpoint with a template_id field.

Before (Postmark):

// POST /email/withTemplate
{
  "TemplateId": 12345,
  "TemplateModel": {
    "name": "User",
    "action_url": "https://example.com/welcome"
  },
  "From": "you@yourdomain.com",
  "To": "user@example.com"
}

After (Postkit):

// POST /v1/emails
{
  "from": "you@yourdomain.com",
  "to": ["user@example.com"],
  "template_id": "tmpl_abc123...",
  "template_data": {
    "name": "User",
    "action_url": "https://example.com/welcome"
  }
}

Add webhook signing

Postmark does not sign webhook payloads -- it relies on Basic Auth and IP allowlisting for security. Postkit uses HMAC-SHA256 signing with a shared secret (prefixed whsec_), following the Standard Webhooks specification.

When you create a webhook in Postkit, you receive a signing_secret. Add signature verification to your webhook handler using the webhook-id, webhook-timestamp, and webhook-signature headers.

See the Webhooks guide for complete verification examples in multiple languages.

Verify your domain

Add your sending domain in Postkit and configure the DNS records (DKIM, SPF, DMARC). Postmark uses "Sender Signatures" and "Domains" -- Postkit unifies domain management under a single /v1/domains endpoint.

See the Domain Setup guide for step-by-step DNS configuration instructions.

Test with sandbox keys

Use a pk_test_ key to verify your integration end-to-end without delivering actual emails. Once everything works as expected, switch to your pk_live_ key for production.