Summary
Email validation services primarily flag domains without MX records as invalid because these records are the fundamental instruction for mail servers on where to deliver incoming mail. While a historical or rarely-used fallback mechanism exists where mail servers might attempt delivery to a domain's A record, this method is generally unreliable and deprecated in modern email ecosystems. Consequently, emails sent to domains lacking MX records are highly likely to result in delivery failures, hard bounces, or spam classification, making reliable email delivery without proper MX records largely impossible.
Key findings
- MX Records as Primary: MX records are the definitive DNS entries that specify which mail servers are configured to receive email for a domain. They are the primary mechanism for Mail Transfer Agents (MTAs) to locate the correct destination for incoming mail.
- Validation Logic: Email validation services flag domains without MX records as invalid because the absence of these records explicitly signals that the domain is not set up to accept incoming emails or lacks proper mail routing instructions.
- Modern Delivery Behavior: Despite a historical fallback to a domain's A record, modern mail servers overwhelmingly rely on MX records. Without them, emails sent to such domains will almost certainly result in hard bounces, delivery failures, or classification as spam.
- Required for Major Providers: Leading email service providers like Google Workspace (Gmail) and Microsoft 365 explicitly require correctly configured MX records for a domain to receive emails through their platforms.
- Undeliverable Status: The lack of an MX record means there is no clear destination for email, rendering the domain unmailable and leading to consistent delivery failures.
Key considerations
- Historical A Record Fallback: While some historical or less common mail server configurations might attempt delivery to a domain's A record when an MX record is absent, this practice is largely deprecated and unreliable in modern email ecosystems. Relying on this fallback is not advised for consistent deliverability.
- Validation Service Perspective: Email validation services flagging domains without MX records as invalid is a perfectly valid and cautious response. It indicates a deviation from standard, reliable email infrastructure, even if a rare or non-standard delivery attempt might theoretically occur.
- Engagement Data vs. Validation: For specific cases involving non-standard configurations, relying on actual engagement data, such as bounces, clicks, and opens, can provide a more accurate assessment of a recipient's validity than solely depending on initial flags from validation services.
What email marketers say
10 marketer opinions
Email validation services primarily flag domains lacking MX records as invalid because these records are indispensable for directing incoming email. Without properly configured MX records, mail servers cannot reliably determine the correct destination for messages, leading to a high probability of delivery failure. While an older, less common practice involved mail servers attempting delivery to a domain's A record in the absence of an MX record, this method is largely deprecated and results in unreliable delivery or messages being flagged as spam by contemporary mail systems.
Key opinions
- MX Records are Definitive: MX records serve as the definitive DNS entries that instruct sending servers where to route incoming mail for a domain.
- Validation for Mailability: Email validation services assess the presence of MX records to determine if a domain is configured to receive emails, marking those without them as invalid or unmailable.
- Modern Delivery Failures: Contemporary mail servers overwhelmingly rely on MX records; without them, emails sent to such domains will almost certainly result in hard bounces, rejections, or spam classification, making reliable delivery impossible.
- Absence Signals Unreadiness: The lack of an MX record explicitly signals that a domain is not properly set up to accept email, leading sending servers to abandon delivery attempts.
- Critical for Transactional Mail: For services relying on timely and assured delivery, such as transactional emails, robust DNS configurations, including MX records, are non-negotiable.
Key considerations
- Deprecated A Record Fallback: While historically some mail servers might have attempted delivery to a domain's A record if no MX record was found, this is not a standard or reliable practice today, often leading to delivery failure.
- Validation as a Best Practice: Email validation services flag missing MX records as a crucial check, indicating a fundamental misconfiguration for email receipt, which is a necessary step for maintaining list hygiene and deliverability.
- Unreliable Non-Standard Setups: Domains configured without MX records, even if theoretically capable of receiving mail via an A record in niche scenarios, pose a significant risk to email deliverability and are generally considered unmailable by the broader email ecosystem.
Marketer view
Email marketer from Email Geeks explains that email can be delivered to a domain even without an MX record; if an MX record is absent, mail servers will attempt delivery to the domain's A record. This is a valid configuration often used by web hosting providers. He adds that email validation services might flag such domains as "invalid" because they are either badly programmed or overly cautious about non-standard setups.
18 Jul 2023 - Email Geeks
Marketer view
Email marketer from ZeroBounce Blog explains that email validation services flag domains without MX records as invalid because the absence of an MX record indicates the domain is not configured to receive emails. A domain must have at least one valid MX record to direct incoming mail to the correct server. Without this crucial record, emails cannot be delivered to the domain, as sending servers would not know where to send them.
25 Nov 2023 - ZeroBounce Blog
What the experts say
3 expert opinions
MX records are the essential guide for email delivery, directing sending mail servers to the correct destination for incoming messages. This fundamental role explains why email validation services classify domains without them as invalid; such domains are not properly configured to receive mail. While a rare, historical fallback exists where some servers might attempt delivery to a domain's A record, this is not a reliable or widely supported method, making emails to these addresses largely undeliverable in today's email landscape.
Key opinions
- MX Records are Foundational: MX records are fundamental for any domain to receive emails, providing the necessary routing instructions for sending mail servers to deliver messages correctly.
- Validation Reflects Standard: Email validation services are accurate in flagging domains without MX records as invalid, as this absence signifies a deviation from standard, mail-receiving configurations.
- A Record as Unreliable Fallback: While a rare, historical precedent allows some mail servers to attempt delivery to a domain's A record in the absence of an MX record, this is not a guaranteed or widely supported method for email delivery today.
- Absence Implies Undeliverability: Without a proper MX record, a domain is considered unconfigured to receive mail, meaning associated email addresses are largely undeliverable by contemporary mail systems.
- Engagement Data is Ultimate Indicator: For non-standard or theoretically mail-accepting setups, actual engagement data-including bounces, clicks, and opens-offers a more reliable indicator of recipient validity than a validation service's initial flag.
Key considerations
- Importance of MX Configuration: Proper MX record setup is paramount for ensuring consistent and reliable email deliverability, as it prevents messages from being rejected due to misconfigured routing.
- Role of Validation Services: Email validation services play a crucial role by identifying domains that deviate from standard mail-receiving configurations, which is vital for maintaining list hygiene and deliverability.
- Unreliability of A Record Delivery: Relying on an A record fallback for email delivery is highly unreliable and should not be considered a viable strategy for effective email marketing campaigns due to its limited support and historical nature.
- Prioritize Real-World Deliverability: For specific or non-standard configurations, actual deliverability and engagement metrics, such as bounces and opens, serve as the most definitive proof of a recipient's ability to receive mail.
Expert view
Expert from Email Geeks shares that some mail servers will accept email on the A record when an MX record is missing, acting as a backup. He notes that while this is possible, the mere presence of an A record doesn't guarantee mail acceptance. He recommends that relying on actual engagement data, such as bounces, clicks, and opens, provides a more accurate way to determine a recipient's validity than validation services. He also clarifies that validation services warning about a missing MX record is a "perfectly valid response" as it indicates a deviation from standard domain configuration.
6 Jun 2025 - Email Geeks
Expert view
Expert from Word to the Wise explains that email validation services flag domains without MX records as invalid because such domains are not configured to receive mail. An email address associated with a domain lacking MX records is considered undeliverable by these services, meaning emails cannot be delivered to them.
8 Feb 2022 - Word to the Wise
What the documentation says
4 technical articles
For reliable email delivery, MX records are absolutely critical, serving as the primary directive for mail servers to locate a domain's mail exchange. This is why email validation services classify domains without them as invalid; such domains fundamentally lack the necessary configuration to receive mail. Although a historical, non-standard fallback to a domain's A record might theoretically exist, this method is largely unreliable and does not facilitate consistent email delivery in contemporary environments.
Key findings
- MX Records: Essential Router: MX records are the fundamental DNS entries that explicitly instruct mail servers where to deliver incoming email for a given domain.
- Validation Reflects Unmailable Status: Email validation services mark domains without MX records as invalid because such domains lack the essential configuration to receive emails, effectively making them unmailable.
- Delivery Fails for Major Platforms: Leading email providers such as Google Workspace, Gmail, and Microsoft 365 mandate correct MX record configuration for a domain to accept mail, failing delivery if they are missing.
- A Record Fallback: Deprecated: The historical practice of mail servers attempting to deliver email to a domain's A record in the absence of MX records is unreliable, non-standard, and generally results in delivery failures today.
- Absence of Designated Server: Without MX records, a domain has no designated mail server, which fundamentally prevents email from being routed or accepted via standard SMTP protocols.
Key considerations
- Prioritize Proper MX Setup: Ensuring correct MX record configuration is paramount for any domain intending to receive email, as it directly impacts deliverability and avoids routing failures.
- Validation for Hygiene: Email validation services serve a vital function by flagging domains without MX records, helping users maintain clean mailing lists and avoid sending to unmailable addresses.
- A Record Fallback Unreliable: Despite its historical existence, attempting email delivery via a domain's A record when MX records are absent is an unreliable and deprecated method that should not be depended upon.
- Impact on Email Ecosystem: The absence of MX records signifies a non-standard setup that major email providers and the broader email ecosystem will largely reject, leading to consistent delivery failures.
Technical article
Documentation from RFC 5321 - Simple Mail Transfer Protocol explains that email validation services flag domains without MX records as invalid because the MX record is the primary mechanism for a Mail Transfer Agent (MTA) to locate the mail exchange server for a recipient domain. While a fallback to an A record lookup exists if no MX records are present, this is not the preferred or reliable method, and often leads to delivery failures, meaning emails generally cannot be reliably delivered without MX records.
6 Feb 2023 - RFC 5321 - Simple Mail Transfer Protocol
Technical article
Documentation from Google Workspace Admin Help explains that for emails to be delivered to a domain hosted on Google Workspace (Gmail), correct MX records must be configured in the domain's DNS settings. If these records are missing or incorrect, email delivery will fail, meaning emails cannot be delivered to such domains.
15 Mar 2023 - Google Workspace Admin Help
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