Does email delivery still fall back to A records if no MX record is published?
Michael Ko
Co-founder & CEO, Suped
Published 6 Jul 2025
Updated 28 Sep 2025
7 min read
The question of whether email delivery still falls back to a domain's A record when no MX record is published is a fascinating one, rooted in the foundational specifications of SMTP. While the RFCs historically allowed for this fallback mechanism, the landscape of email deliverability has evolved significantly. I've seen firsthand how crucial proper DNS configuration is in ensuring messages reach their intended recipients, and a lack of MX records introduces considerable uncertainty.
For email marketers and system administrators, understanding this nuance is vital. Relying on an A record fallback in today's email environment can lead to significant deliverability issues, including bounces and impact on sender reputation. Let's dive into the technical details and real-world implications.
The role of MX records in email delivery
MX records, or Mail Exchanger records, are a fundamental component of the Domain Name System (DNS) specifically designed to direct email traffic for a domain. They tell sending mail servers which mail servers are responsible for accepting email messages on behalf of your domain and, importantly, their priority.
When an email is sent, the sending server performs a DNS lookup to find the MX records for the recipient's domain. If multiple MX records exist, they are ordered by a preference value, with lower numbers indicating higher priority. The sending server attempts to connect to the highest-priority mail server first. This process ensures that mail is routed correctly and can failover if a primary mail server is unavailable. You can learn more about DNS MX records explained to understand their role fully.
A typical MX record looks something like this, indicating the hostname of the mail server and its priority:
Example MX recorddns
example.com. IN MX 10 mail.example.com.
Without an MX record, the sending server lacks explicit instructions on where to deliver mail, leading to ambiguity and potential delivery failures. It's a critical piece of the puzzle for email deliverability.
The A record fallback mechanism
The original Simple Mail Transfer Protocol (SMTP) specification, particularly RFC 5321 (previously RFC 2821), states that if no MX records are published for a domain, the sending server should attempt to deliver mail directly to the host specified by the domain's A record. This was a sensible fallback in the early days of the internet when many smaller domains might have run their mail server directly on their web server, identified only by an A record.
Historically, this fallback was crucial for vanity web hosting or small personal domains that might not have had a dedicated MX record but still needed to receive email. If the server associated with the A record was configured to listen on port 25 (the standard SMTP port) and accept mail, then delivery could, in theory, succeed. This approach worked well enough for simpler setups where the A record pointed to a machine capable of handling email.
RFC expectation
Direct delivery: If no MX records are found, SMTP servers attempt to deliver email to the IP address specified by the domain's A record.
Simplicity: A simplified setup for domains where web and mail services reside on the same server.
Modern reality
Low reliability: Most modern mail servers reject or bounce emails sent to A records without MX records.
Security concerns: Increased risk of spam filtering and delivery issues due to non-standard configuration.
However, this fallback mechanism is now largely considered an antiquated practice. The internet has evolved, and email infrastructure has become far more complex and security-conscious. Modern mail servers typically prioritize security and adherence to established best practices, which include the explicit use of MX records for mail routing.
Modern email ecosystem and A record fallback
In the modern email ecosystem, it's generally unsafe to assume that email will be reliably delivered if a domain lacks an MX record, even if it has a valid A record. Many receiving mail servers will simply reject email addressed to domains without properly configured MX records. They view this as a non-standard configuration and a potential red flag for spam or misconfigured domains. The sending system might attempt delivery to the A record, but the receiving server often won't accept it.
Even if an A record points to a server listening on port 25, that server needs to be a fully functional mail server capable of accepting mail for the specific domain, processing it, and placing it into an inbox. This is rarely the case for generic web servers. Most web servers are not configured to act as mail servers, meaning any attempt to deliver mail directly to their A record would fail.
The cost of missing MX records
Attempting to deliver email to domains without proper MX records can be resource-intensive for sending mail servers. Each failed attempt consumes bandwidth, processing power, and time due to DNS lookups and connection attempts that ultimately lead to bounces. This inefficiency can negatively impact your overall sending performance and could even lead to your IPs being placed on a blacklist (or blocklist). It's much better to validate your lists beforehand.
For domains that should receive email, a missing MX record often indicates a misconfiguration or a domain that is not intended for email receipt. Email validation services frequently flag such domains as invalid for good reason. Assuming "no MX equals no delivery" is a safer bet for maintaining good sender reputation than relying on an outdated fallback.
Why relying on A records is a risk
Relying on the A record fallback for email delivery is a significant risk to your email program. It can lead to an inflated bounce rate, which negatively impacts your sender reputation. ISPs and mailbox providers closely monitor bounce rates, and high rates signal that you might be sending to invalid or poorly managed lists, leading to your emails being filtered to spam (or outright rejected) for legitimate recipients.
Additionally, if a domain has no MX record but an A record, there's no guarantee the server at that IP address is secured for email. This could potentially expose messages to security vulnerabilities, although modern sending protocols (like TLS) mitigate some of these risks. The lack of a proper MX record can also complicate troubleshooting when delivery issues arise, as the expected mail flow is not being followed.
For email delivery, robust DNS configuration, including accurate MX records, is paramount. This also extends to other records like SPF and DKIM, which are critical for email authentication and preventing spoofing. Tools like Suped's DMARC monitoring provide insight into the authentication status of your emails, helping you identify and fix issues that could impact deliverability. Ensuring proper DNS setup helps prevent intermittent email delivery failures caused by SPF and DNS issues.
Views from the trenches
Best practices
Always validate email lists to remove domains with missing or incorrect MX records. This reduces bounces and improves sender reputation.
Prioritize explicit MX records for all domains intended to receive email. This aligns with modern email server expectations and security protocols.
Implement DMARC monitoring with Suped to gain visibility into email authentication and delivery issues, including those related to DNS configuration.
Regularly check your DNS records, including MX and A records, to ensure they are correctly configured and updated.
Common pitfalls
Assuming the RFC fallback to an A record is still a reliable delivery mechanism in today's email landscape.
Not cleaning email lists that contain domains with no MX records, leading to high bounce rates and wasted resources.
Relying on email validation services that only check for MX records, potentially discarding valid but non-standard recipients.
Failing to monitor DMARC reports, which can highlight issues with MX records and other authentication failures.
Expert tips
For optimal deliverability, ensure all email-receiving domains have properly configured MX records.
Use comprehensive email validation tools that go beyond simple MX checks to assess domain deliverability.
Monitor your delivery logs for patterns of bounces related to missing MX records to refine your suppression lists.
Consider the target audience;
Expert view
Expert from Email Geeks says that while the sending system may attempt delivery to an A record, many recipient systems will reject mail if no MX record is found, often stating 'Sender domain invalid' in bounces.
2025-09-18 - Email Geeks
Marketer view
Marketer from Email Geeks says that for typical online shoppers on e-commerce mailing lists, it's safe to assume that if there's no MX record, mail will not be delivered reliably.
2025-09-18 - Email Geeks
For the most part, the notion of email delivery reliably falling back to a domain's A record in the absence of an MX record is a relic of older SMTP implementations. While the RFC technically allows for it, modern email systems are far more stringent, prioritizing explicit MX records for security, reliability, and proper routing.
To ensure optimal email deliverability, always ensure your domains have correctly configured MX records. Avoid relying on the A record fallback, as it will likely lead to high bounce rates, damaged sender reputation, and ultimately, emails failing to reach their intended recipients. Proactive DNS management and robust email validation are key to maintaining a healthy email program.
Monitoring your email delivery is critical. Platforms like Suped provide comprehensive DMARC monitoring and reporting, offering the most generous free plan available. This allows you to gain deep insights into your email authentication and ensure your messages are authenticated and delivered as expected. Staying on top of these technical configurations is essential for success in today's demanding email environment.
