Receive Email Webhooks in FastAPI

Quick Start: FastAPI Email Webhook

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

pip install fastapi uvicorn

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

from fastapi import FastAPI, Request

app = FastAPI()

@app.post("/webhook")
async def webhook(request: Request):
    payload = await request.json()
    email = payload["email"]
    print(f"Email from: {email['from']} | Subject: {email['subject']}")
    return {"status": "ok"}

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 FastAPI 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 fastapi import FastAPI, Request, HTTPException
from fastapi.responses import JSONResponse

app = FastAPI()
WEBHOOK_SECRET = os.environ["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.post("/webhook")
async def webhook(request: Request):
    raw_body = await request.body()  # Must read raw bytes first
    sig = request.headers.get("x-jsonhook-signature", "")

    if not verify_signature(raw_body, sig):
        raise HTTPException(status_code=401, detail="Invalid signature")

    try:
        payload = json.loads(raw_body)
    except json.JSONDecodeError:
        raise HTTPException(status_code=400, detail="Invalid JSON")

    event     = payload["event"]
    timestamp = payload["timestamp"]
    address   = payload["address"]
    email     = payload["email"]

    print(f"[{timestamp}] Email at {address} from {email['from']}")
    print(f"Subject: {email['subject']}")

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

    return {"status": "ok"}

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
from dataclasses import dataclass
from typing import List, Optional

@dataclass
class Attachment:
    filename: str
    contentType: str
    size: int
    contentId: Optional[str]

@dataclass
class EmailData:
    from_: str
    to: List[str]
    subject: str
    textBody: str
    htmlBody: str
    attachments: List[Attachment]

# After signature verification:
raw_body = await request.body()
payload = json.loads(raw_body)

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

sender    = email["from"]
subject   = email["subject"]
text_body = email["textBody"]

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 fastapi import Request, HTTPException

WEBHOOK_SECRET = os.environ["JSONHOOK_WEBHOOK_SECRET"].encode()

async def get_verified_body(request: Request) -> bytes:
    """
    Read the raw request body and verify the JsonHook HMAC-SHA256 signature.
    Raises 401 HTTPException on failure.
    """
    raw_body = await request.body()
    sig = request.headers.get("x-jsonhook-signature", "")
    computed = hmac.new(WEBHOOK_SECRET, raw_body, hashlib.sha256).hexdigest()
    if not hmac.compare_digest(computed, sig):
        raise HTTPException(status_code=401, detail="Invalid signature")
    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.

FastAPI-specific tips:

• Always call await request.body() before await request.json() — once you call request.json(), the raw bytes are consumed and cannot be re-read for HMAC computation
• Use hmac.compare_digest() for constant-time comparison; do not use == on HMAC hex strings
• Use FastAPI's BackgroundTasks or an async task queue like Celery or ARQ to process email data after returning the response
• Set uvicorn --timeout-keep-alive 30 and ensure your endpoint returns in under 10 seconds to avoid JsonHook treating the delivery as failed

FastAPI Framework Tips

FastAPI 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 FastAPI 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 FastAPI's built-in request validation or a schema library (e.g., Zod, Pydantic, etc.) to validate the payload structure after signature verification.