Why do email validation services flag domains without MX records as invalid, and can emails still be delivered?
Matthew Whittaker
Co-founder & CTO, Suped
Published 9 Jul 2025
Updated 19 Aug 2025
6 min read
It can be confusing when email validation services flag a domain as invalid because it lacks an MX (Mail Exchange) record, especially if you know for a fact that emails are still being delivered to addresses on that domain. I've encountered this situation countless times, and it often leads to questions about the accuracy of these validation tools and the fundamental rules of email delivery.
The core of this confusion lies in understanding how mail servers (also known as Mail Transfer Agents, or MTAs) locate the correct destination for an email and the different methods they employ. While MX records are the standard and preferred way to direct email traffic, there are legitimate scenarios where their absence doesn't prevent delivery.
The role of MX records in email delivery
An MX record is a type of DNS record that specifies which mail servers are responsible for accepting email messages on behalf of a domain name and where emails for that domain should be routed. When you send an email, the sending mail server performs a DNS lookup to find the MX record for the recipient's domain. This record then directs the email to the appropriate mail server.
For instance, if you send an email to user@example.com, your mail server will query the DNS for example.com's MX records. It will receive a list of mail servers, often with preference values (lower numbers indicating higher preference). The sending server then attempts to connect to these mail servers, starting with the highest priority, to deliver the message.
This mechanism ensures that email is routed efficiently and reliably to the correct destination. Without a properly configured MX record, many standard mail servers would struggle to determine where to send the email, leading to delivery failures.
The A record fallback mechanism
Despite the primary role of MX records, there's a fallback mechanism defined in older internet standards. If a domain does not have any MX records, mail servers are instructed to look for an A record (or AAAA record for IPv6) for the domain itself. If an A record exists, the mail server can then attempt to deliver the email directly to the IP address specified by that A record, assuming that IP also hosts a mail server listening on the standard SMTP port (port 25).
This fallback is outlined in RFC 1912, a common DNS errors document. This is precisely why some domains, particularly smaller, personal domains or those hosted by providers with non-standard configurations, can still receive email even without an explicit MX record.
You can check for the presence of an A record using a simple dig or nslookup command.
Checking for an A recordBASH
dig example.com a
or
nslookup -query=a example.com
Why validation services flag missing MX records
Email validation services, in their effort to provide accurate deliverability assessments, often adopt a strict interpretation of email standards. They primarily check for the presence of MX records because this is the universally accepted and most reliable way a domain indicates its readiness to receive email. When an MX record is missing, even if an A record exists, many validators flag the email address as invalid or unknown. Some consider this an overly cautious approach.
The reason for this strictness is often rooted in risk management. While an A record fallback technically allows delivery, it's not the primary or most robust way to configure a mail server. Domains relying solely on A records for mail delivery might be less stable, less secure, or more prone to unexpected issues. Validation services aim to help you maintain a clean and effective email list, and including potentially unreliable addresses could undermine that goal.
Therefore, when a validation service flags such an email, it's typically indicating a deviation from standard best practices rather than a guaranteed inability to deliver. It's a signal that while mail might be deliverable, it's not configured in the most optimal or universally compatible way.
Validator's perspective
Standard check: Primarily looks for MX records to confirm mail-receiving capability.
Risk assessment: Flags domains without MX records as potentially unreliable or non-standard, even with an A record.
Goal: To minimize bounce rates and protect sender reputation by advising against sending to such addresses.
Actual email delivery
MX preference: Mail servers first attempt to use MX records for routing.
A record fallback: If no MX records are found, an A record for the domain can act as a mail server if configured to receive mail on port 25.
Deliverability: Emails can still be delivered, especially to personal domains or specific hosting setups.
Practical implications for deliverability
While an email address without an MX record but with an A record might technically be deliverable, sending to such addresses carries increased risks. Mail servers that strictly adhere to modern RFCs (Request for Comments) or have advanced spam filters might reject emails to domains without explicit MX records. This can lead to higher bounce rates, which negatively impacts your sender reputation.
A high bounce rate signals to Internet Service Providers (ISPs) that your sending practices might be poor, potentially leading to your IP or domain being added to a blocklist (or blacklist). Being on a blocklist can severely hinder your ability to reach inboxes, affecting all your future email campaigns. Therefore, even if some emails get through, the overall health of your email program is at stake.
Best practices for email list hygiene
Maintaining a clean email list is crucial for deliverability, regardless of whether a domain has an MX record. Regularly validate your email lists to remove invalid, inactive, or risky addresses. While a flagged domain might still receive mail, the associated risks to your sender reputation often outweigh the benefit of keeping that address on your list. Focus on engaged subscribers who consistently open and click your emails, as this is the most reliable indicator of a truly deliverable address.
Conclusion
Email validation services flag domains without MX records as invalid because it deviates from the standard and most reliable method for email routing. While emails can still be delivered to such domains via an A record fallback, it introduces a level of uncertainty and potential deliverability issues.
For optimal email deliverability and to protect your sender reputation, it's generally best to avoid sending to addresses on domains that lack proper MX records. Prioritize maintaining a clean and engaged email list, focusing on subscribers whose domains are correctly configured for reliable email receipt.
Views from the trenches
Best practices
Always prioritize maintaining a clean and verified email list to improve overall deliverability and sender reputation.
Use engagement metrics, such as opens and clicks, as a strong indicator of a valid and active email recipient.
For critical communications, consider alternative channels if a recipient's email domain shows unusual DNS configurations.
Common pitfalls
Over-reliance on email validation services without understanding their underlying logic for flagging domains.
Ignoring addresses flagged as 'unknown' or 'risky' by validation services, assuming they are always deliverable.
Sending high volumes of emails to domains lacking MX records, which can lead to increased bounce rates and blocklisting.
Expert tips
If you have a list of domains without MX records, test the deliverability to a small segment before a full send.
Consider that domains without MX records might be personal or niche, potentially requiring a different engagement strategy.
Educate your sales and marketing teams on why some valid-looking emails might be flagged by validation tools.
Expert view
Expert from Email Geeks says you do not strictly need an MX record for email delivery, as a domain with an A record can often act as a mail server if no MX record is found.
2019-09-30 - Email Geeks
Marketer view
Marketer from Email Geeks says that web hosting providers sometimes configure domains to deliver email directly to the A record, bypassing the need for an explicit MX record.
