Validating Email Flows on a Zero-Budget: API Development for Security Researchers

Ensuring reliable email flow validation is crucial for maintaining the integrity of communication systems, especially in security-sensitive applications. For security researchers operating under tight budget constraints, traditional tools and paid services might not be feasible. This post explores how to develop a simple, yet effective, API solution to validate email flows — all without spending a dime.

The Challenge

In many security scenarios, verifying that email flows are correctly configured and functioning is a foundational step. These validations include:

• Ensuring email deliverability
• Checking SPF, DKIM, and DMARC records
• Confirming proper routing and forwarding

Traditional solutions involve commercial services or complex server setups, which are often resource-intensive or costly. The goal here is to architect an API-centric approach leveraging free tools and open-source resources.

Step 1: Define the Core Functionalities

The API should:

• Accept email addresses or domains as input
• Perform DNS record checks for SPF, DKIM, DMARC
• Send a test email and verify receipt
• Report the validation status with detailed insights

Step 2: Leveraging Free Tools and APIs

To develop this, we can utilize free DNS querying libraries, such as dig or dns.resolver in Python, combined with free email testing services.

DNS Record Validation

We'll use Python's dns.resolver to query DNS records:

import dns.resolver

def check_dns_records(domain):
    records = {}
    try:
        records['SPF'] = dns.resolver.resolve(domain, 'TXT')
        records['DKIM'] = dns.resolver.resolve(f"_domainkey.{domain}", 'TXT')
        records['DMARC'] = dns.resolver.resolve(f'_dmarc.{domain}', 'TXT')
    except dns.resolver.NoAnswer:
        pass
    except dns.resolver.NXDOMAIN:
        return None
    return records

This function retrieves SPF, DKIM, and DMARC records, essential for email validation.

Sending and Verifying Test Emails

While sending actual emails might seem complex without dedicated servers, we can leverage free SMTP testing services like Mailtrap (free tier), or even utilize Python's smtplib to send emails through an existing SMTP server (e.g., Gmail, if available). For zero budget, deploying a lightweight SMTP server locally or on a free cloud platform (like Render or Railway) can suffice.

import smtplib
from email.mime.text import MIMEText

def send_test_email(smtp_server, port, sender, receiver, message):
    msg = MIMEText(message)
    msg['Subject'] = 'Test Email'
    msg['From'] = sender
    msg['To'] = receiver

    with smtplib.SMTP(smtp_server, port) as server:
        server.starttls()
        server.login(sender, 'password')  # Use environment variables for credentials
        server.send_message(msg)

API Architecture

Using Flask (a lightweight Python web framework), we can compose these functionalities into an API:

from flask import Flask, request, jsonify
app = Flask(__name__)

@app.route('/validate-email', methods=['POST'])
def validate_email():
    data = request.json
    domain = data.get('domain')
    email = data.get('email')
    results = {}
    # DNS Checks
    dns_results = check_dns_records(domain)
    results['dns'] = dns_results if dns_results else 'No DNS records found'
    # Email send/receipt verification can be added here if email credentials are available
    # Placeholder for sending email
    # results['send_status'] = 'Success' or 'Failure'
    return jsonify(results)

if __name__ == '__main__':
    app.run(debug=True)

Step 3: Automation and Continuous Validation

Automate the API to run periodically or trigger via webhook integrations. For example, initiate checks whenever DNS records are updated or email routing changes occur.

Final Remarks

This approach exemplifies how security researchers can develop effective validation tools without any commercial dependency or sizable expenditure. The key lies in leveraging open-source libraries, free services, and thoughtful API design to create a resilient system that enhances email security posture. By making validation processes part of automated security workflows, organizations can proactively identify misconfigurations and potential threats.

Remember, continuous updates and security-aware coding practices are vital, especially when deploying these tools in real environments. While this setup is meant for research and testing, it offers a solid foundation for scalable, budget-conscious email validation solutions.

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🛠️ QA Tip

To test this safely without using real user data, I use TempoMail USA.