Why do email validation services flag domains without MX records as invalid, and can emails still be delivered?

Key findings

• A record fallback: While MX records are the primary mechanism for directing email, mail servers will attempt to deliver email to a domain's A record if no MX record is found. This is a fallback mechanism, not the standard. You can learn more about whether an A record is needed for email sending domains.
• Validation service caution: Many email validation services flag domains without MX records as 'unknown' or 'invalid' because this setup deviates from standard email infrastructure configurations. This cautious approach helps prevent bounces.
• Deliverability impact: Despite being technically deliverable, sending to domains without proper MX records can still negatively affect your sender reputation or lead to higher bounce rates with ESPs that strictly adhere to MX record requirements.
• User engagement as validation: For domains with recent engagement (opens, clicks), their deliverability is confirmed by user behavior, even if validation tools flag them. As Mailgun highlights, email verification is vital for deliverability, but engagement offers real-world proof.

Key considerations

• Balance validation and engagement: Do not discard valid, engaged contacts simply because a validation service flags them due to a missing MX record. Your internal engagement data provides a more accurate picture.
• Understand service logic: Recognize that validation services err on the side of caution. Their primary goal is to minimize hard bounces, and a missing MX record often signals a higher risk. This is why understanding email validation tools and practices is essential.
• Monitor for bounces: If you choose to send to such domains, closely monitor your bounce rates. High bounce rates can lead to blocklists and damage your sender reputation.
• Consider manual verification: For critical contacts, manual checks (like seeing if TCP port 25 is open on the domain's A record) can provide additional assurance, but proceed cautiously to avoid triggering spam filters.

Key opinions

• Validation vs. reality: Marketers frequently encounter situations where validation services report an email as invalid or unknown, despite concrete evidence of recent engagement (opens, clicks) from that address. This discrepancy creates a challenge in list management.
• Trusting engagement data: Many marketers believe that actual engagement data, such as recent opens or clicks, is a more reliable indicator of an email's validity than a validation service's assessment, especially for addresses associated with missing MX records. This aligns with advice that valid MX records are important, but real-world interaction is paramount.
• ESP behavior variation: There's a recognized curiosity among marketers about whether different ESPs (Email Service Providers) will attempt delivery to a domain with no MX record but a valid A record. Experiences show that some ESPs do, while others might not.
• Risk aversion in validation: Marketers understand that validation services are designed to be cautious, flagging any deviation from standard DNS configurations (like missing MX) to protect sender reputation, even if mail could technically be delivered. Learn more about how external domains can affect sender reputation.

Key considerations

• Prioritize engaged segments: For email addresses from domains without MX records, if there is recent, verifiable engagement, marketers should consider these contacts as valid recipients, despite validation service flags.
• Review ESP policies: Marketers should investigate how their specific ESP handles email delivery to domains that lack MX records but have a valid A record to understand potential deliverability issues.
• Segment questionable contacts: Consider placing contacts from such domains into a separate, closely monitored segment to track deliverability and engagement, mitigating risks to overall sender reputation.
• Educate on DNS: Marketers should be aware of the basic DNS record types like MX and A records, and how they influence email delivery, to better interpret validation results and troubleshoot deliverability issues.

Key opinions

• A record as mail server: Experts confirm that an MX record is not strictly mandatory for email delivery. If a domain has an A record but no MX record, mail servers will attempt to deliver directly to the IP address specified by the A record. This behavior is detailed further in guides about A records for return-path domains.
• Lookup hierarchy: The MX record is the first point of lookup for email routing. Only if an MX record is absent will the system then attempt to use the A record for delivery, indicating a fallback mechanism rather than a primary method.
• Validation service design: Experts suggest that email validation services may flag domains without MX records as invalid or risky because this setup is a deviation from the standard, recommended configuration, regardless of actual deliverability. This is because email validation helps ensure deliverability.
• Common configuration: Some web hosting providers, particularly for personal or small domains, configure their client domains to deliver directly to the A record without an explicit MX record. This is a valid, albeit non-standard, setup.

Key considerations

• Best practices: While technically possible, relying on A record fallbacks for email delivery is not considered a best practice for reliable and consistent deliverability. Proper MX records are always recommended.
• Port 25 checks: To verify if a domain without an MX record can indeed receive email via its A record, one can check if TCP port 25 is listening on that IP address. However, this should be done in very low volumes to avoid being flagged as suspicious, which could lead to being listed on a blocklist.
• Domain configuration quality: A domain lacking proper MX records may indicate a poorly configured setup, especially if it's a personal domain. Such configurations could potentially lead to other deliverability challenges.
• Reputation implications: Sending to domains with non-standard configurations could subtly impact your sender reputation, as some receiving mail servers might be more stringent in their checks.

Key findings

• MX record purpose: MX records (Mail Exchanger records) are a type of DNS record that specifies which mail servers are responsible for accepting email messages on behalf of a domain name. They also indicate the preference (priority) of those mail servers.
• A record as fallback: RFC 5321 (the standard for email transfer) states that if a domain has no MX records, the mail server should attempt to deliver mail directly to the host specified by the domain's A record (address record).
• Validation logic: Email validation services typically check for the presence of MX records as a primary step to verify a domain's ability to receive email. The absence of an MX record may lead to a 'risky' or 'unknown' classification due to this deviation from standard practice.
• Implicit mail handling: Some hosting environments implicitly configure their primary web server's A record to also handle mail if no explicit MX record is set up for the domain, making it technically capable of receiving emails. This is touched upon in technical documentation about email validators.

Key considerations

• Reliability concern: While RFCs allow A record fallback, relying on this method can be less reliable than using explicit MX records, as some older or stricter mail servers might not consistently apply this fallback, potentially leading to bounces or delays. This ties into the broader topic of what RFCs say versus what actually works.
• Standard vs. practice: The RFC defines a standard, but real-world implementation by various mail servers and email validation services can vary, leading to different interpretations of a domain's mail-receiving capability.
• DNS configuration impact: Proper DNS configuration, including MX records, is fundamental for email deliverability and authentication protocols like SPF, DKIM, and DMARC. A missing MX record can complicate these authentications and overall deliverability. Learn more about DMARC, SPF, and DKIM.
• Security implications: While not directly stated, non-standard configurations could sometimes be associated with less secure or less professionally managed domains, which may attract more scrutiny from receiving mail servers.