How to Test Postmark Webhooks

This guide covers everything you need to test Postmark webhooks — how to inspect the raw payload, verify the signature, forward events to localhost, and reproduce any delivery in your local development environment without needing a real Postmark event.

How to Test Postmark Webhooks with WebhookWhisper

1. Create a free endpoint — click Create Live Endpoint above to get a permanent public HTTPS URL (no account required to try)
2. Register the URL in Postmark — paste the WebhookWhisper URL into Postmark's webhook settings
3. Trigger a test event — use the one-click test payload sender or trigger a real event in Postmark
4. Inspect the request — see the full headers, raw body, and Postmark signature header in real time
5. Forward to localhost — add a forwarding rule to relay the event to your local handler (e.g. http://localhost:3000/webhooks/postmark)

Postmark Webhook Signature Verification

Postmark signs webhook deliveries using HMAC-SHA256. The signature is sent in the X-Postmark-Signature header. Always verify this signature before processing the payload to ensure the request came from Postmark and was not tampered with in transit.

Node.js Verification

const crypto = require('crypto')
const express = require('express')
const app = express()

app.post('/webhooks/postmark',
  express.raw({ type: 'application/json' }),
  (req, res) => {
    const secret = process.env.POSTMARK_WEBHOOK_SECRET
    const signature = req.headers['x-postmark-signature']

    // Compute expected HMAC — always use raw body, never parsed JSON
    const expected = crypto
      .createHmac('sha256', secret)
      .update(req.body)
      .digest('hex')

    if (!crypto.timingSafeEqual(Buffer.from(signature || ''), Buffer.from(expected))) {
      return res.status(401).json({ error: 'Invalid signature' })
    }

    const event = JSON.parse(req.body)
    // Process event here — respond first, process async for slow operations
    res.json({ received: true })
  }
)

Python (FastAPI) Verification

import hashlib, hmac, os
from fastapi import FastAPI, Request, HTTPException

app = FastAPI()

@app.post('/webhooks/postmark')
async def webhook(request: Request):
    secret = os.environ['POSTMARK_WEBHOOK_SECRET'].encode()
    raw_body = await request.body()
    signature = request.headers.get('x-postmark-signature', '')

    expected = hmac.new(secret, raw_body, hashlib.sha256).hexdigest()

    if not hmac.compare_digest(signature, expected):
        raise HTTPException(status_code=401, detail='Invalid signature')

    payload = await request.json()
    return {'received': True}

Common Postmark Webhook Errors

Forward Postmark Webhooks to Localhost

Use WebhookWhisper to receive Postmark webhook events at a public HTTPS URL and relay them to your local development server — no tunnel, no CLI install, no public server required.

1. Create a WebhookWhisper endpoint and paste it into Postmark's webhook settings
2. In the Forwarding tab, set target URL to http://localhost:3000/webhooks/postmark
3. Every Postmark event appears in the inspector and hits your local handler simultaneously
4. Use event replay (Pro) to re-send any captured event without triggering a new action in Postmark

FAQ

Do I need a Postmark account to test webhooks?

No. WebhookWhisper includes a one-click Postmark sample payload so you can fire a realistic test event and verify your handler without a Postmark account or triggering a real Postmark action.

How do I find my Postmark webhook secret?

The webhook signing secret is shown in Postmark's developer settings or webhook configuration page. Each webhook endpoint gets its own secret — do not share secrets between endpoints.

What is the Postmark webhook timeout?

Most providers timeout after 5–30 seconds. If your handler does slow operations (database writes, external API calls), respond with HTTP 200 immediately and process the event in a background job to avoid triggering Postmark's retry logic.