What are some examples of common but unusual SPF and MX records?
Matthew Whittaker
Co-founder & CTO, Suped
Published 28 Apr 2025
Updated 16 Aug 2025
8 min read
When managing email infrastructure, most of us focus on setting up standard Sender Policy Framework (SPF) and Mail Exchanger (MX) records. These foundational DNS records are critical for email authentication and routing, ensuring your messages reach their intended recipients and defending against spoofing.
However, the world of DNS records is vast, and sometimes you encounter configurations that are common in their existence but highly unusual in their structure or intent. These can be the result of misconfigurations, attempts at overly clever solutions, or specific niche use cases.
Understanding these uncommon patterns is essential for troubleshooting deliverability issues, identifying potential security vulnerabilities, and maintaining a robust email ecosystem.
SPF records are published as TXT records in your domain's DNS and specify which mail servers are authorized to send email on behalf of your domain. While most SPF records are straightforward, some exhibit truly head-scratching qualities. One common mistake I often see involves including private IP ranges like ip4:10.0.0.0/8 or the localhost address ip4:127.0.0.1. These IPs are not routable on the public internet, making their inclusion redundant and indicative of a misunderstanding of SPF's purpose. It often stems from a misconception that SPF should reflect every machine that *might* touch an email, rather than just those sending it externally.
Another unusual but surprisingly frequent scenario is an SPF record that technically looks valid but contains so many include mechanisms that it exceeds the 10-DNS-lookup limit imposed by the SPF specification. This leads to SPF validation failures, as receiving mail servers stop processing the record after the limit is hit. I’ve seen records with dozens of include statements for every conceivable email service a company has ever used, even if they no longer send from them. You can learn more about this by checking out the SPF 10-DNS-lookup limit.
Another peculiar, albeit less common, issue is including URL links or other non-standard text directly within the SPF record string, as seen in the example above. An SPF record must adhere to specific syntax rules to be parsed correctly. Anything outside the defined mechanisms and qualifiers can render the record invalid, leading to email authentication failures. Such errors can severely impact your domain reputation and inbox placement.
Best practices for SPF records
Regular audits: Periodically review your SPF records to ensure they only include active, authorized sending sources.
Avoid private IPs: Never include internal IP addresses or localhost in your public SPF records.
Strict syntax: Always adhere to the official SPF syntax to prevent parsing errors.
Decoding puzzling MX records
MX records, or Mail Exchanger records, tell the internet which mail servers are responsible for accepting incoming email for a particular domain. While straightforward in concept, some configurations can be misleading or indicate a specific, often unusual, intent. A classic example is setting an MX record to localhost or 127.0.0.1. While this might seem like a misconfiguration, it's often a deliberate choice for domains that do not intend to receive email. It effectively tells other mail servers, 'Don't bother, we don't accept mail here.' This is a perfectly valid setup for domains used solely for sending, or for those whose email services are hosted elsewhere under a different subdomain.
Another less common but intentional MX record is the Null MX record, which points to a single dot .. Defined in RFC 7505, this explicitly states that the domain does not accept email. This is useful for domains that exist purely for branding or as part of a larger security strategy to prevent abuse, for example, for domains that do not send email. While less intuitive than localhost, it serves a similar function with a clearer standard.
Sometimes you also encounter MX records with extremely low or high priority values that don't align with a typical primary/backup setup, or multiple MX records pointing to the same server. These can indicate a lack of understanding of MX priority, or perhaps an attempt to distribute load that isn't working as intended. For instance, if all records have the same priority, mail will be delivered in an unpredictable order, or possibly even randomly split between servers.
Normal MX setup
Typically, you'll see multiple MX records with varying priority numbers, directing mail to a primary server first, and then to backup servers if the primary is unavailable. This ensures reliability and redundancy.
Unusual setups might include pointing to non-existent hosts, internal IPs, or using the 'null MX' record or 'localhost' address to explicitly deny incoming mail. These can be intentional or a sign of misconfiguration.
The impact of unusual or misconfigured SPF and MX records can range from minor annoyances to catastrophic deliverability and security failures. When an SPF record is poorly constructed, legitimate emails might fail SPF authentication checks. This often leads to messages being flagged as spam, quarantined, or outright rejected by receiving mail servers. For example, if you include private IP ranges or exceed the 10-DNS-lookup limit, your emails might not authenticate correctly, affecting your inbox placement.
On the security front, SPF failures can weaken your domain's defense against email spoofing and phishing attempts. While SPF alone isn't a silver bullet, it's a crucial layer. A broken SPF record makes it easier for malicious actors to send emails impersonating your domain, potentially leading to brand damage and financial losses. This is particularly true if your DMARC policy relies heavily on SPF alignment, as a misconfigured SPF record will cause DMARC authentication to fail. I've seen situations where even with an SPF record in place, a domain was vulnerable due to incorrect entries.
For MX records, unusual configurations primarily affect inbound email flow. If your MX records point to non-existent servers, internal IPs, or are otherwise misdirected, incoming emails will bounce back to the sender. This can result in missed communications, lost business, and a perception of unreliability. Even if a null MX or localhost MX is intentional, failing to properly communicate that intent or using it incorrectly for a domain that *should* receive mail can cause significant issues.
The risks of incorrect DNS records
Email deliverability: Legitimate emails may be marked as spam or rejected, affecting communication and business operations.
Domain reputation: Consistent authentication failures can harm your sending reputation, leading to further deliverability problems and potential blocklisting.
Security vulnerabilities: Misconfigured SPF can make your domain susceptible to spoofing and phishing attacks, compromising trust and security. You can find out more by reading an advanced guide to email authentication.
Missed communications: Incorrect MX records lead to undeliverable inbound emails, impacting customer service and business continuity.
Rectifying misconfigurations
Rectifying unusual or problematic SPF and MX records requires a systematic approach. The first step is always to gain a clear understanding of all legitimate email sending sources and receiving points for your domain. This involves auditing your email service providers, marketing automation platforms, transactional email services, and any in-house mail servers. For SPF, ensure that every authorized sender is accurately represented in your record and that you avoid exceeding the 10-DNS-lookup limit.
For MX records, confirm that they point to the correct, publicly accessible mail servers for incoming email. If your domain isn't meant to receive email, consider explicitly using a Null MX record MX 0 . to clearly communicate this intent and avoid confusion or delivery attempts. After making any changes, always use a DNS checker to verify the records have propagated correctly and are free of errors.
Ongoing monitoring is key. Even perfectly configured records can become problematic if your sending infrastructure changes or new services are adopted. Regularly review your DMARC reports, as these provide invaluable feedback on SPF and DKIM authentication results, helping you spot issues early. proactive management of your DNS records is crucial for consistent email deliverability and robust security. For more assistance, see how to troubleshoot email delivery failures.
Views from the trenches
Best practices
Always include all legitimate sending IPs and `include` mechanisms in SPF.
Regularly audit DNS records to reflect current sending infrastructure.
Utilize Null MX records (e.g., `MX 0 .`) for domains that don't receive email to prevent confusion.
Ensure SPF records adhere strictly to RFC syntax to avoid parsing issues and authentication failures.
Common pitfalls
Exceeding the 10-DNS-lookup limit in SPF records, which causes authentication failures.
Including internal IP ranges or `localhost` in public SPF records, leading to unnecessary complexity.
Having multiple MX records with identical priorities when load balancing is not intended.
Failing to update DNS records after changes to email service providers or infrastructure.
Expert tips
SPF private network ranges and multicast ranges are ignored by the SPF specification, so including them is usually pointless.
A common reason for incorrect SPF entries is misinterpreting email headers, leading IT staff to add `localhost` to their SPF records.
Ensure your DMARC policy aligns with your SPF and DKIM configurations; otherwise, emails may still fail delivery.
If your domain doesn't send email, ensure it has a proper SPF record, typically `v=spf1 -all`, along with a Null MX record.
Marketer view
Marketer from Email Geeks says a record had 42 includes or IP addresses but failed to include the three actual IPs emails were being sent from.
2023-02-03 - Email Geeks
Expert view
Expert from Email Geeks says an MX record pointing to localhost is common and acceptable for domains that do not accept mail, as is an MX record pointing to a single dot.
2023-02-03 - Email Geeks
Maintaining a robust email infrastructure
Navigating the complexities of SPF and MX records can be challenging, especially when encountering unusual configurations. While some unusual setups are intentional for specific purposes like indicating a domain does not receive mail, many are the result of misconfigurations that can severely impact email deliverability and expose your domain to security risks. Failing SPF authentication or bouncing incoming mail due to incorrect MX records can damage your reputation and disrupt communication.
A proactive approach, including regular audits, adherence to best practices, and leveraging monitoring tools, is essential. By meticulously managing your DNS records, you can ensure your emails consistently reach their recipients, protect your domain from abuse, and maintain a robust and trustworthy email infrastructure.
