How can I lookup and categorize MX records for a large list of email addresses?
Michael Ko
Co-founder & CEO, Suped
Published 11 May 2025
Updated 18 Aug 2025
6 min read
Managing large email lists presents a unique set of challenges, especially when it comes to ensuring your messages actually reach the inbox. A crucial aspect of maintaining strong email deliverability is understanding the underlying infrastructure of the email addresses on your list, specifically their Mail Exchange (MX) records. These records act like signposts, guiding incoming mail to the correct servers.
For individual lookups, tools are readily available, but when you're dealing with hundreds of thousands or even millions of email addresses, a different approach is necessary. Bulk MX record lookup and categorization allow you to gain valuable insights into your audience's email providers, helping you tailor your sending strategies and ultimately boost your inbox placement rates.
The basics of MX records
MX records are a fundamental component of the Domain Name System (DNS), dedicated to specifying the mail servers responsible for receiving email on behalf of a domain. When you send an email, the sending server performs a DNS query to find the recipient's MX record. This record then directs the email to the appropriate mail server.
A domain can have multiple MX records, each with a priority value. The server attempts to deliver email to the server with the lowest priority number first. If that server is unavailable, it moves to the next lowest priority. This redundancy ensures email delivery even if a primary mail server experiences issues. Understanding these priority values can sometimes reveal backup or third-party mail services.
Knowing the MX record of an email address's domain provides direct insight into which mail server, and thus which email service provider, is handling that email. This knowledge is incredibly powerful for optimizing your sending practices. For example, if you know a significant portion of your list uses Google Workspace, you can align your sending behavior with Google's email sender guidelines to improve your deliverability to those recipients.
Example: Looking up MX records for a domainbash
dig MX example.com
Why categorize your email list by MX record?
Categorizing your email list by MX record provides a strategic advantage in email marketing and deliverability. It shifts your focus from individual email addresses to the infrastructure behind them, allowing for a more nuanced approach to list management and campaign optimization.
This categorization helps you understand the composition of your audience based on their email providers. Different providers (Outlook, Yahoo, ProtonMail, etc.) have unique filtering algorithms, spam trap networks, and sender reputation systems. By segmenting your list, you can identify potentially harmful domains or those that pose a higher risk of bounces or blocklisting (blacklisting).
This intelligence enables you to proactively manage your sender reputation and avoid hitting spam traps or being placed on blocklists. For instance, if you see a high percentage of addresses pointing to domains known for aggressive spam filtering, you might adjust your sending volume or content for that segment. It's a key step in maintaining a clean email list.
The importance of list hygiene
A dirty email list can significantly harm your sender reputation and lead to reduced deliverability. MX record categorization helps identify potential issues before they cause widespread problems.
Identify defunct domains: If a domain has no MX record, it likely cannot receive email.
Spot invalid records: Malformed MX records can lead to delivery failures.
Flag free email providers: While not inherently bad, these often have stricter filtering.
Methods for bulk MX record lookups
For a large list of email addresses, manual MX record lookups are simply not feasible. You need an automated approach. This typically involves scripting or utilizing specialized tools designed for bulk DNS queries.
One common method involves writing a script in a language like Python, PowerShell, or Bash. These scripts can read a list of domains (extracted from your email addresses), perform DNS queries for each domain's MX record, and then store the results. Libraries like `dnspython` in Python simplify this process significantly.
For very large datasets, performance is key. Direct system calls to `dig` or `nslookup` can be slow when scaled up. Optimized DNS resolution libraries or specialized tools that can handle asynchronous queries are preferred. Some open-source projects, like the dbdnsresolve tool mentioned in the search results, are built specifically for fast bulk DNS resolution against databases, demonstrating the need for highly efficient solutions.
Manual lookup (for small lists)
Process: Use online tools like DNS Checker MX lookup or command-line utilities for one-off checks.
Pros: Simple for quick verifications, no setup required.
Cons: Impractical and time-consuming for large lists, lacks automation.
Automated bulk lookup (for large lists)
Process: Develop scripts using DNS libraries or leverage specialized bulk checking tools.
Pros: Efficient for millions of records, enables systematic data analysis.
Cons: Requires technical expertise for setup and maintenance.
Categorizing and leveraging MX data
Once you have collected the MX records for your large list of domains, the next crucial step is categorization. This is where the raw data transforms into actionable intelligence for your email strategy. Categorization often involves mapping MX records to their respective email service providers or identifying common MX patterns.
For example, many domains may use Microsoft 365 or Google Workspace for their email hosting. While their specific MX records might differ slightly, they will often point to a common parent domain like outlook.com or google.com in their MX hostname. This allows you to group related domains and apply specific deliverability strategies for each major provider.
Categorization can also help you identify suspicious MX records, such as those associated with known spam traps or disreputable providers. By understanding these patterns, you can make informed decisions about segmenting your list, adjusting sending volumes, and prioritizing engagement efforts for optimal inbox placement. For more details on this, you can look up how to get email providers from MX records.
Provider
Common MX record pattern
Example MX
Google Workspace
*.google.com
aspmx.l.google.com
Microsoft 365 (Exchange Online)
*.protection.outlook.com
example-com.mail.protection.outlook.com
Yahoo Mail
*.yahoodns.net
mx-apac.mail.gm0.yahoodns.net
ProtonMail
*.protonmail.ch
mail.protonmail.ch
Boosting deliverability with MX insights
Effectively looking up and categorizing MX records for a large list of email addresses is a powerful technique for any sender committed to optimizing email deliverability. It's more than just a technical exercise; it's a strategic move that provides granular insights into your audience's email infrastructure.
By understanding which mailbox providers serve your recipients, you can proactively adapt your sending practices, mitigate risks, and ensure your legitimate emails consistently reach the inbox rather than falling into spam folders or being blocklisted. This level of data-driven list management is essential for maintaining a healthy sender reputation and achieving successful email campaigns.
Views from the trenches
Best practices
Always validate email addresses before attempting MX lookups to reduce unnecessary queries and improve data accuracy.
Use asynchronous or multithreaded approaches for bulk lookups to maximize efficiency and minimize processing time.
Map MX records to common email service providers (ISPs) and categorize your list based on these providers to tailor sending strategies.
Common pitfalls
Attempting to perform bulk MX lookups using simple, synchronous scripts which can be extremely slow for large lists.
Not accounting for rate limits or query limits when interacting with DNS servers, leading to blocked lookups.
Failing to properly categorize MX records by the actual mailbox provider, thus missing critical deliverability insights.
Expert tips
When dealing with very large lists, consider distributing the lookup tasks across multiple machines or using cloud-based DNS resolution services.
Beyond basic MX categorization, analyze the sub-domains or specific hostnames within MX records to identify more granular service providers or custom setups.
Track changes in MX records for your most important domains to detect shifts in email hosting providers, which might signal changes in filtering policies.
Expert view
Expert from Email Geeks says they have developed code specifically for bulk MX record lookups.
2020-09-23 - Email Geeks
Expert view
Expert from Email Geeks says their code cleans and categorizes data by MX owner because different domains can share the same underlying mail exchange infrastructure.
Outlook, 
Yahoo, 
ProtonMail, etc.) have unique filtering algorithms, spam trap networks, and sender reputation systems. By segmenting your list, you can identify potentially harmful domains or those that pose a higher risk of bounces or blocklisting (blacklisting).
Microsoft 365 or 
Google Workspace for their email hosting. While their specific MX records might differ slightly, they will often point to a common parent domain like outlook.com or google.com in their MX hostname. This allows you to group related domains and apply specific deliverability strategies for each major provider.
