How to build an email allow list using MX records for initial email validation?
Matthew Whittaker
Co-founder & CTO, Suped
Published 17 May 2025
Updated 16 Aug 2025
8 min read
Building an effective email allow list is a critical step in maintaining a clean and deliverable email list. While many focus on dynamic real-time validation, strengthening your initial filters can significantly reduce the amount of 'garbage' data entering your system. One powerful, yet sometimes overlooked, method for this first-level validation involves leveraging MX (Mail Exchange) records.
Traditional allow lists often rely on pre-defined lists of common domains. However, the internet is constantly evolving, with new domains and subdomains emerging regularly. This makes maintaining a static allow list a never-ending and often futile task. Consider how many variations of major email providers exist across different countries and top-level domains, for example, yahoo.co.uk or outlook.com.au. Manually compiling and updating such a list is inefficient and prone to errors, often leading to legitimate email addresses being incorrectly flagged.
The core problem is that a domain name alone doesn't guarantee email deliverability or legitimacy. An email address might have correct syntax, but if its domain doesn't have proper MX records, emails simply cannot be delivered to it. This is where MX records come in, offering a more robust and dynamic approach to initial email validation and allow list creation.
The foundational role of MX records
An MX record (Mail Exchange record) is a type of DNS record that specifies which mail servers accept incoming mail for a domain and where email messages sent to that domain should be routed. Think of it as a signpost that tells the internet where to send emails for a particular address. Without a valid MX record, an email cannot be received by a domain.
The primary way to use MX records for initial validation is to perform an MX lookup for any new email address domain you receive. If a domain lacks an MX record, it's a strong indicator that the email address is invalid or won't be able to receive mail. This immediately allows you to filter out a significant portion of malformed or non-functional email addresses before they even hit your more complex real-time validation systems. This also helps in avoiding spam traps and other undesirable addresses.
Beyond simply checking for the presence of an MX record, you can also analyze the MX record's content. Major email providers like Google, Microsoft, and Yahoo (and AOL, which uses Yahoo's MX infrastructure) have predictable MX record patterns. By identifying these patterns, you can build a dynamic allow list that recognizes domains hosted by reputable email providers, regardless of their specific top-level domain.
The risk of ignoring MX records
Failing to validate MX records can lead to several problems, impacting your email deliverability and sender reputation. If you're interested in deeper email validation specifics, remember that a domain without an MX record is typically a dead end for email.
Higher bounce rates: Sending emails to addresses on domains without MX records will result in hard bounces, damaging your sender reputation.
Wasted resources: Every invalid email you try to send consumes resources and slows down your email sending process.
Spam trap risk: Some domains without MX records might actually be spam traps, designed to catch senders with poor list hygiene.
Building an MX-based allow list strategy
Instead of compiling static lists of domains, an MX-based allow list focuses on identifying the underlying mail infrastructure. The logic is simple: if a domain's MX record points to a recognized, legitimate mail server (like Google Workspace, Outlook, or Yahoo Mail), it's likely a valid and deliverable email address. This approach is far more scalable and adaptable than trying to keep up with every single domain name.
To implement this, for each new email address submission, you'd extract the domain, perform an MX lookup, and then check the resolved MX records against a predefined list of known, reputable mail server hostnames. For example, if a domain's MX record points to something like mail.protection.outlook.com, you can confidently add that domain to your allow list for initial validation. This process is often part of more comprehensive email list validation strategies.
A crucial optimization for this method is caching. MX records don't change frequently, so once you've looked up a domain's MX records, you can store this information for a set period, like 24 hours or even a week. This significantly speeds up subsequent validations for the same domain, reducing the load on your DNS lookups and making your initial filter more efficient.
Static domain allow list
Maintenance: Requires constant manual updating or reliance on outdated third-party lists.
Accuracy: Prone to missing new legitimate domains or including outdated ones.
Scalability: Does not scale well with the vast and dynamic nature of email domains globally.
MX record-based allow list
Maintenance: Dynamically identifies legitimate domains by checking against known MX patterns, reducing manual effort.
Accuracy: More resilient to new domains and subdomains as long as they use standard mail providers.
Scalability: Scales effectively by focusing on provider infrastructure rather than individual domains.
Practical implementation and considerations
To perform an MX record lookup, you can use various tools or programming libraries. Command-line tools like dig (Linux/macOS) or nslookup (Windows) are common. Here's an example using dig:
Example MX lookup with digbash
dig MX example.com
The output will show one or more MX records with priority values. You'll then need to parse these results and compare the hostnames to your list of known, trusted mail server patterns. This is part of a broader set of DNS-based email filtering techniques.
One important consideration is data cleanup. When processing MX records, ensure you standardize the format, such as lowercasing everything and identifying common root domains. For instance, mx0.google.com and mx1.google.com should be recognized as belonging to the same provider, Google. This involves developing robust parsing logic or utilizing existing libraries that handle these nuances.
While this method is powerful, it's not foolproof. Some domains, particularly smaller ones or those with custom email setups, might have valid MX records that don't fit common patterns. Also, be wary of long-tail domains or domains that appear to be from a major provider but actually point to something else (e.g., a yahoo.ms type domain that doesn't actually belong to Yahoo). These could potentially be spam traps or invalid addresses. For additional methods, consider exploring recommended email validation tools.
Advanced techniques and challenges
The scale of MX record lookups can be a significant challenge, especially if you're attempting to validate large existing lists or continuously process massive volumes of sign-ups. Resolving all .com, .net, and .org domains, for example, can take days or even weeks if done serially. This is why caching is so important.
Another advanced technique involves monitoring how often MX records change for various domains. While individual lookups are generally quick, understanding the churn rate of MX records can help optimize your caching strategy and identify potentially unstable domains. Some services even try to do this at scale for broad internet domains, though it is a colossal task. For context on this challenge, Microsoft's documentation on DNS records highlights the complexity.
The MX record-based allow list is a powerful first line of defense, but it is typically combined with other email validation techniques, such as syntax checks, disposable email address detection, and sometimes even SMTP server pinging (though this method has its own pitfalls and is often debated for its efficacy and potential for potential issues). This multi-layered approach ensures comprehensive email hygiene.
Views from the trenches
Best practices
Implement a caching mechanism for MX records to optimize lookup speed and reduce repeated DNS queries for common domains.
Regularly update your known MX patterns for major email providers, as these can occasionally change or expand.
Combine MX record validation with other checks, like syntax validation, to build a robust multi-layered email validation system.
Prioritize MX lookups for new sign-ups to ensure that initial data collected is as clean as possible, reducing processing of invalid emails.
Common pitfalls
Relying solely on a static list of domains for an allow list, as it quickly becomes outdated and misses new legitimate domains.
Not accounting for 'long-tail' domains or subdomains that might have legitimate MX records but don't fit common patterns.
Overlooking domains that appear to be from major providers but have MX records pointing elsewhere, potentially indicating a spam trap.
Failing to implement proper data cleanup, such as lowercasing or normalizing MX record hostnames, which can lead to missed matches.
Expert tips
The Public Suffix List (PSL) can be helpful in identifying common roots for domain cleanup and classification.
Consider serializing DNS lookups for very large datasets to avoid potential UDP packet loss issues, which can occur with parallel queries.
Even if a domain appears to be from a major provider like Yahoo, always verify its MX records, as some lookalike domains are designed as traps.
For very large-scale domain analysis, consider leveraging cloud infrastructure to perform lookups, though it can still be a resource-intensive task.
Expert view
Expert from Email Geeks says: You can look up and cache MX records. They generally do not change very often, so a cache refresh every 24 hours or even a week should be sufficient for most needs.
March 7, 2023 - Email Geeks
Marketer view
Marketer from Email Geeks says: Maintaining a static list of domains for an allow list is challenging because domain lists are constantly changing and expanding, making it difficult to keep up manually.
March 7, 2023 - Email Geeks
Conclusion
Building an email allow list based on MX records provides a dynamic and robust first line of defense for email validation. By shifting from static domain lists to recognizing the underlying mail infrastructure, you can significantly improve the accuracy of your initial filters. This proactive approach helps reduce bounces, conserve resources, and protect your sender reputation by keeping invalid or potentially harmful email addresses off your list.
While MX record validation is powerful, it is most effective when integrated into a comprehensive email hygiene strategy that includes other validation methods and continuous monitoring. A multi-layered approach ensures your email list remains clean, engaged, and ready for optimal deliverability.
yahoo.co.uk or 
outlook.com.au. Manually compiling and updating such a list is inefficient and prone to errors, often leading to legitimate email addresses being incorrectly flagged.
Google, 
Microsoft, and 
Yahoo (and AOL, which uses Yahoo's MX infrastructure) have predictable MX record patterns. By identifying these patterns, you can build a dynamic allow list that recognizes domains hosted by reputable email providers, regardless of their specific top-level domain.
