Receive Email Webhooks in Flask

Quick Start: Flask Email Webhook

JsonHook delivers every inbound email as a JSON POST request to your webhook endpoint. Setting up a Flask handler takes less than 5 minutes. Start by initializing your project:

pip install flask

Then create your webhook endpoint. The following example shows the minimal code needed to receive and acknowledge a JsonHook delivery:

from flask import Flask, request

app = Flask(__name__)

@app.route('/webhook', methods=['POST'])
def webhook():
    payload = request.get_json()
    print('From:', payload['email']['from'])
    print('Subject:', payload['email']['subject'])
    return '', 200

if __name__ == '__main__':
    app.run(port=3000)

Point your JsonHook address webhook URL to this endpoint and you will start receiving parsed emails as JSON within seconds of the email arriving.

Full Flask Implementation

The quick start example above is enough to get started, but a production implementation should include signature verification, structured error handling, and proper HTTP response codes. The complete example below demonstrates all of these patterns together.

This implementation verifies the X-JsonHook-Signature header to confirm the request genuinely came from JsonHook, parses the full email payload, and returns the appropriate HTTP status codes to trigger or suppress retries.

import hmac
import hashlib
import json
import os
from flask import Flask, request, jsonify, abort

app = Flask(__name__)
WEBHOOK_SECRET = os.environ.get('JSONHOOK_WEBHOOK_SECRET', '').encode()

def verify_signature(raw_body: bytes, sig_header: str) -> bool:
    if not sig_header:
        return False
    computed = hmac.new(WEBHOOK_SECRET, raw_body, hashlib.sha256).hexdigest()
    return hmac.compare_digest(computed, sig_header)

@app.route('/webhook', methods=['POST'])
def webhook():
    raw_body = request.get_data()  # Must call before get_json()
    sig = request.headers.get('X-JsonHook-Signature', '')

    if not verify_signature(raw_body, sig):
        app.logger.warning('Invalid signature')
        abort(401)

    payload = json.loads(raw_body)
    email   = payload['email']

    app.logger.info(f"Email from {email['from']}: {email['subject']}")

    for att in email.get('attachments', []):
        app.logger.info(f"Attachment: {att['filename']} ({att['size']} bytes)")

    return '', 200

if __name__ == '__main__':
    app.run(port=3000, debug=False)

The webhook handler returns 200 immediately after queuing the email for processing. Avoid doing expensive work (database writes, API calls) synchronously inside the handler — process the payload in a background job to stay within JsonHook's 10-second response timeout.

Parsing the Webhook Payload

Every JsonHook delivery is an HTTP POST with Content-Type: application/json. The payload follows a consistent schema regardless of the originating email client or provider:

import json

raw_body = request.get_data()
payload = json.loads(raw_body)

event     = payload['event']      # "email.received"
timestamp = payload['timestamp']
address   = payload['address']

email     = payload['email']
from_addr = email['from']
to        = email['to']
subject   = email['subject']
text_body = email['textBody']
html_body = email.get('htmlBody', '')

for att in email.get('attachments', []):
    print(att['filename'], att['contentType'], att['size'])

Key fields in the payload:

• event — Always "email.received" for inbound email events
• timestamp — ISO 8601 timestamp of when JsonHook received the email
• address — The JsonHook inbound address that received the email (e.g., [email protected])
• email.from — Sender address string, e.g., "Alice <[email protected]>"
• email.to — Array of recipient address strings
• email.subject — Email subject line
• email.textBody — Plain text body of the email (may be empty if HTML-only)
• email.htmlBody — HTML body of the email (may be empty if plain-text-only)
• email.attachments — Array of attachment objects, each with filename, contentType, size, and contentId

Verifying Webhook Signatures

JsonHook signs every webhook delivery using HMAC-SHA256. The signature is included in the X-JsonHook-Signature request header as a hex digest. To verify it, compute the HMAC-SHA256 of the raw request body using your address's webhook secret and compare it to the header value.

Your webhook secret is returned when you create an inbound address via the API (POST /api/addresses). Store it as an environment variable — never hard-code it.

import hmac
import hashlib
import os
from flask import request, abort

WEBHOOK_SECRET = os.environ.get('JSONHOOK_WEBHOOK_SECRET', '').encode()

def verify_jsonhook_signature():
    """Call at the start of your Flask webhook view."""
    raw_body = request.get_data()
    sig = request.headers.get('X-JsonHook-Signature', '')
    computed = hmac.new(WEBHOOK_SECRET, raw_body, hashlib.sha256).hexdigest()
    if not hmac.compare_digest(computed, sig):
        abort(401)
    return raw_body

Always verify the signature before processing the payload. Return 401 for invalid signatures so that legitimate retries from JsonHook (which always include a valid signature) are distinguishable from spoofed requests.

Error Handling Best Practices

Reliable webhook handling requires careful attention to error responses. JsonHook uses your HTTP response code to decide whether to retry a delivery:

• Return 200 quickly: Acknowledge receipt immediately and process asynchronously. JsonHook will retry any non-2xx response.
• Return 400 for bad requests: If the payload fails your own validation (not signature — use 401 for that), return 400 to prevent retries of malformed deliveries.
• Return 500 to trigger retries: If your downstream system is temporarily unavailable, returning 500 causes JsonHook to retry with exponential backoff (up to 5 attempts over ~1 hour).
• Never return 200 before verifying the signature: Doing so silently accepts spoofed requests.

Flask-specific tips:

• Call request.get_data() before request.get_json() in Flask — the body stream can only be read once
• Use hmac.compare_digest() not == for signature comparison
• Use flask.after_this_request or a Celery task to defer heavy processing after returning the 200 response
• Set PROPAGATE_EXCEPTIONS = True in Flask config during development to see full tracebacks; disable it in production to prevent leaking internals

Flask Framework Tips

Flask provides several conveniences that make webhook handling cleaner. Here are framework-specific patterns to use when integrating JsonHook:

• Register your webhook route before any authentication middleware — the JsonHook request does not carry user credentials, only the HMAC signature.
• Use raw body access for signature verification. Many Flask frameworks parse the body automatically — make sure you are hashing the raw bytes, not the re-serialized parsed object.
• Use a dedicated route or controller file for webhook handlers to keep the codebase organized as you add more inbound address integrations.
• Log the address field from every payload to track which inbound address received the email — useful for multi-address setups.
• Consider using Flask's built-in request validation or a schema library (e.g., Zod, Pydantic, etc.) to validate the payload structure after signature verification.