Email validation for Python + FastAPI

Arcjet lets you validate and verify an email address. This is useful for preventing users from signing up with fake email addresses and can significantly reduce the amount of spam or fraudulent accounts.

What is Arcjet?

Arcjet is the runtime security platform that ships with your code. Enforce budgets, stop prompt injection, detect bots, and protect personal information with Arcjet's AI security building blocks.

Quick start

This guide shows you how to add email validation and verification to your app.

1. Install Arcjet

In your project root, install the SDK:

In your project root, install the Arcjet Python SDK along with FastAPI and Uvicorn:

uv add arcjet fastapi uvicorn

If you don’t use uv, pip install arcjet fastapi uvicorn (or your preferred package manager’s equivalent) works too.

2. Set your key

Create a free Arcjet account then follow the instructions to add a site and get a key. Add it to a .env.local file in your project root.

Add your key to a .env file in your project root. Arcjet does not read ARCJET_KEY automatically – your application must pass it explicitly to arcjet(key=...).

.env

# Run Arcjet in development <https://docs.arcjet.com/environment#arcjet-env>.
ARCJET_ENV=development
# Arcjet key for your site (from <https://console.arcjet.com>).
# More info: <https://docs.arcjet.com/environment#arcjet-key>.
ARCJET_KEY=ajkey_yourkey

Install python-dotenv and call load_dotenv() before reading the environment, or use FastAPI’s pydantic-settings integration which loads .env automatically.

3. Validate an email address

The following example shows how to use Arcjet to check an email address. If the email address is invalid or if no MX records are configured, Arcjet returns a deny decision. Pass exactly one of allow or deny. An empty allow list allows no email types.

Create a new file at main.py with the contents:

main.py

import os

from arcjet import EmailType, Mode, arcjet, validate_email
from fastapi import FastAPI, Form, Request
from fastapi.responses import JSONResponse

app = FastAPI()

aj = arcjet(
    key=os.environ["ARCJET_KEY"],  # Get your site key from https://console.arcjet.com
    rules=[
        validate_email(
            mode=Mode.LIVE,  # Blocks requests. Use Mode.DRY_RUN to log only
            # Block disposable, invalid, and email addresses with no MX records
            deny=[EmailType.DISPOSABLE, EmailType.INVALID, EmailType.NO_MX_RECORDS],
        ),
    ],
)


@app.post("/")
async def index(request: Request, email: str = Form(...)):
    print("Email received:", email)

    decision = await aj.protect(request, email=email)
    print("Arcjet decision", decision)

    if decision.is_denied():
        # The email rule denied this request — return 400 because the email is
        # invalid rather than 403 which implies the user is forbidden.
        if decision.reason_v2.type == "EMAIL":
            return JSONResponse({"error": "Invalid email"}, status_code=400)
        return JSONResponse({"error": "Forbidden"}, status_code=403)

    return {"message": "Hello world", "email": email}

4. Start server

Start the FastAPI dev server:

uv run uvicorn main:app --reload

Make a curl POST request from your terminal to your application with various emails to test the result. Arcjet denies a disposable address with a 400 Bad Request:

curl -X POST -d 'email=test@0zc7eznv3rsiswlohu.tk' http://localhost:8000/

The requests also appear in the Arcjet dashboard.

FAQs

Do I need to run any infrastructure e.g. Redis?

No, Arcjet handles all the infrastructure for you so you don't need to worry about deploying global Redis clusters, designing data structures to track rate limits, or keeping security detection rules up to date.

What is the performance overhead?

Arcjet SDK tries to do as much as possible asynchronously and locally to minimize latency for each request. Where decisions can be made locally or previous decisions are cached in-memory, latency is usually <1ms.

When a call to the Cloud API is required, such as when tracking a rate limit in a serverless environment, there is some additional latency before a decision is made. The Cloud API has been designed for high performance and low latency, and is deployed to multiple regions around the world. The SDK will automatically use the closest region which means the total overhead is typically no more than 20-30ms, often significantly less.

What happens if Arcjet is unavailable?

Where a decision has been cached locally e.g. blocking a client, Arcjet will continue to function even if the service is unavailable.

If a call to the Cloud API is needed and there is a network problem or Arcjet is unavailable, the default behavior is to fail open and allow the request. You have control over how to handle errors, including choosing to fail close if you prefer. See the reference docs for details.

How does Arcjet protect me against DDoS attacks?

Network layer attacks tend to be generic and high volume, so these are best handled by your hosting platform. Most cloud providers include network DDoS protection by default.

Arcjet sits closer to your application so it can understand the context. This is important because some types of traffic may not look like a DDoS attack, but can still have the same effect. For example, a customer making too many API requests and affecting other customers, or large numbers of signups from disposable email addresses.

Network-level DDoS protection tools find it difficult to protect against this type of traffic because they don't understand the structure of your application. Arcjet can help you to identify and block this traffic by integrating with your codebase and understanding the context of the request e.g. the customer ID or sensitivity of the API route.

Volumetric network attacks are best handled by your hosting provider. Application level attacks need to be handled by the application. That's where Arcjet helps.

What next?

Email validation reference Full details about email validation.

Testing Write tests for your rules.

Explore

Arcjet can be used with specific rules on individual routes or as general protection on your entire application. You can set up bot protection, rate limiting for your API, minimize fraudulent registrations with the signup form protection and more.

Example apps See the example apps.

Learn how Arcjet works Arcjet's architecture.

SDK reference Python SDK reference guide.

Get help

Need help with anything? Email support@arcjet.com to get support from our engineering team.

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