How to Connect Auth0 with MailerCheck (2026)

Why Connect Auth0 and MailerCheck

Auth0 is a flexible identity and access management platform that provides authentication, authorization, and user management for applications. It handles login flows, multi-factor authentication, social login, passwordless authentication, and enterprise identity federation. Auth0 serves as the identity layer for thousands of applications, managing millions of user accounts and login events.

MailerCheck is an email verification and deliverability platform that validates email addresses to ensure they are real, active, and safe to send to. It detects invalid addresses, disposable emails, spam traps, and role-based addresses through both bulk list verification and real-time API checks. MailerCheck helps businesses maintain clean email lists and protect their sender reputation.

Connecting Auth0 and MailerCheck adds an email verification layer to your authentication and user management workflow. By verifying email addresses at the point of registration through Auth0, businesses can prevent fake signups, reduce bot registrations, and ensure that every user account is tied to a real, deliverable email address. This improves both security and the quality of your user database.

What This Integration Does

Integrating Auth0 with MailerCheck brings email verification into the identity management process. Here is what this connection enables:

• Registration Email Verification: Validate email addresses through MailerCheck in real time during the Auth0 signup process, rejecting disposable or invalid emails before accounts are created.
• User Database Cleaning: Run existing Auth0 user email lists through MailerCheck's bulk verification to identify and flag accounts with invalid, disposable, or risky email addresses.
• Fraud Prevention: Block registration attempts using known disposable email domains or spam trap addresses, reducing fraudulent account creation.
• Deliverability Protection: Ensure that transactional emails sent through your application, such as password resets and account notifications, reach valid addresses by pre-verifying at signup.
• User Quality Scoring: Enrich Auth0 user profiles with MailerCheck quality scores, helping you identify high-quality users versus potentially fake or low-engagement accounts.

Native Integration vs Third-Party

Auth0 and MailerCheck do not have a native integration. Auth0 provides a powerful extensibility framework through Actions (formerly Rules and Hooks) that allow custom logic to run during authentication and registration flows. MailerCheck offers a REST API for real-time email verification that can be called from these Auth0 extensibility points.

The most effective approach for real-time registration verification is a direct API integration using Auth0 Actions. By writing a small Auth0 Action that calls MailerCheck's API during the pre-registration flow, you can verify emails without any third-party automation tool. For bulk verification of existing users, Zapier or Make can orchestrate periodic exports from Auth0 to MailerCheck. n8n provides a self-hosted option for handling bulk verification workflows. The Auth0 Action approach is recommended for real-time checks due to its speed and reliability.

Step-by-Step Setup

Step 1: Get MailerCheck API Access

Sign up for a MailerCheck account and navigate to the API settings to generate your API key. Review the API documentation for the single email verification endpoint, noting the request format, response fields, and rate limits. Ensure your MailerCheck plan includes sufficient API calls for your expected registration volume.

Step 2: Create an Auth0 Action

In your Auth0 dashboard, navigate to Actions and create a new Action in the Pre User Registration flow. This Action will execute before a new user account is created, giving you the opportunity to verify the email address and block the registration if the email fails verification. Name the Action descriptively, such as "MailerCheck Email Verification."

Step 3: Write the Verification Logic

In the Auth0 Action editor, write the code that calls MailerCheck's API with the registering user's email address. Parse the response to check the email status. If MailerCheck reports the email as invalid, disposable, or a spam trap, deny the registration with an appropriate error message. Store your MailerCheck API key as an Auth0 Action secret rather than hardcoding it.

Step 4: Test the Integration

Test the Auth0 Action using Auth0's Action testing tools and by attempting registrations with different email types. Test with valid emails, disposable email addresses, invalid syntax, and known bad domains. Verify that valid emails are accepted and invalid ones are rejected with clear, user-friendly error messages.

Step 5: Deploy and Monitor

Deploy the Action to your production Auth0 tenant. Monitor the Action logs in Auth0 to track verification results and ensure the MailerCheck API is responding within acceptable timeframes. Set up alerts for high error rates or API timeouts. Also consider running a one-time bulk verification of your existing user database through MailerCheck to clean up historical data.

Common Use Cases

• SaaS Registration Quality: Verify email addresses during SaaS application signup through Auth0, preventing free trial abuse from users registering with disposable emails.
• Marketplace Account Integrity: Ensure that marketplace users register with valid email addresses, reducing fake accounts and improving the trust level of the user community.
• Transactional Email Deliverability: By verifying emails at registration, ensure that critical transactional messages like order confirmations, password resets, and account alerts reach valid inboxes.
• Lead Quality for B2B Applications: Filter out registrations with personal or disposable emails for B2B applications, ensuring the user database contains contacts reachable for sales and marketing outreach.
• Existing User Database Cleanup: Run your current Auth0 user database through MailerCheck bulk verification to identify dormant accounts with invalid emails, improving data quality and reducing costs.

Tips and Best Practices

• Implement the MailerCheck API call with a timeout in your Auth0 Action. If MailerCheck does not respond within two to three seconds, allow the registration to proceed rather than blocking users due to API latency.
• Cache MailerCheck verification results for recently checked domains to reduce API calls and speed up the registration flow for users from the same email provider.
• Be selective about which MailerCheck statuses you block. Blocking "invalid" and "disposable" is usually safe, but blocking "risky" or "unknown" may reject legitimate users with unusual email configurations.
• Provide clear, helpful error messages when blocking registrations. Tell users their email could not be verified and suggest using a different email address, rather than displaying a generic error.
• Monitor your MailerCheck API usage against your plan limits, especially during traffic spikes. Running out of API credits would effectively block all new registrations if not handled properly.
• Run bulk verification of your Auth0 user database quarterly to maintain data quality, catching email addresses that have become invalid since the user registered.
• Store the MailerCheck verification status as metadata in the Auth0 user profile for future reference, allowing you to audit and report on the quality of your user database.