Summary
Intermittent email delivery failures, particularly those linked to Sender Policy Framework (SPF) and Domain Name System (DNS) issues, often stem from a combination of complex SPF records, DNS resolution inconsistencies, and occasional recipient-side challenges. Common culprits include SPF records that exceed the 10 DNS lookup limit or the 255-character string limit, as well as DNS caching issues, slow DNS responses, or improper DNS propagation. While some problems originate on the receiving server's end, senders can mitigate many of these issues by meticulously validating their SPF records, ensuring correct DNS configurations, and monitoring their domain's DNS health.
Key findings
- SPF Record Complexity: A primary cause of intermittent SPF failures is an SPF record that is too long or too complex, leading to excessive DNS lookups (exceeding the 10-lookup limit) or exceeding the 255-character string limit, which some mail servers reject.
- DNS Resolution Issues: Frequent causes include DNS caching problems at the recipient's end, inconsistent DNS propagation, slow or timed-out responses from the sender's DNS servers, and issues with dynamic IP addresses not being properly updated in the SPF record.
- Recipient-Side Factors: Intermittent delivery failures can be exacerbated by the recipient's mail server configuration, such as overly aggressive SPF lookup timeouts, underspecified local DNS resolvers, or transient network issues on their end, which are generally beyond the sender's control to fix directly.
- Incorrect DNS Records: Missing or incorrectly configured DNS records, including MX, DKIM, and especially PTR records for sending IPs, can contribute to intermittent delivery issues and SPF failures.
- Multiple SPF Records: Having more than one SPF TXT record for a domain is a common misconfiguration that leads to unpredictable and intermittent SPF validation outcomes.
Key considerations
- Validate SPF Record: Thoroughly check your SPF record for syntax errors, ensure it adheres to the 255-character string limit, and critically, that it does not exceed the 10 DNS lookup limit. Utilize SPF lookup tools to validate the record, verify all included mechanisms resolve correctly, and confirm all legitimate sending services and IP addresses are explicitly authorized and current. Consider flattening your SPF record where possible to reduce lookup counts.
- Monitor DNS Health: Regularly monitor your domain's DNS for slow responses, timeouts, and propagation issues. Ensure your DNS provider is correctly publishing your SPF and other critical records like MX, DKIM, and PTR. Be mindful that DNS changes can take up to 48 hours to fully propagate globally. If making changes, consider temporarily reducing TTL (Time To Live) for DNS records before changes and reverting it afterward to minimize update times.
- Check for Multiple SPF Records: A common misconfiguration is having multiple SPF records for the same domain, which can cause intermittent failures. Ensure your domain only has one valid SPF TXT record.
- Understand DNS Caching: Intermittent failures often stem from DNS caching issues at the recipient's end, where an old SPF record might still be served by some DNS resolvers. While largely out of your control, ensuring prompt DNS updates and managing TTLs can help.
- Review PTR Records: Ensure your outbound IP addresses have valid reverse DNS (PTR) records that correctly match your sending domain, as some receiving servers perform these lookups as an anti-spam measure.
- Utilize Diagnostic Tools: Employ DNS lookup tools and DMARC reporting to identify specific SPF validation failures and pinpoint problematic sending sources or DNS inconsistencies. Monitoring mail logs for specific error codes related to SPF can also provide valuable insights.
- Address Local Network Issues: In some cases, local router or firewall DNS settings might interfere with correct DNS resolution for outgoing email. Temporarily bypassing local DNS settings can help diagnose if the issue originates from your immediate network environment.
What email marketers say
12 marketer opinions
Troubleshooting intermittent email delivery failures, especially those tied to SPF and DNS, requires a systematic approach. Such issues often arise from a confluence of factors, including the complexity of the sender's SPF record, inconsistencies in DNS resolution and propagation, and potential challenges on the recipient's mail server or network. While senders can address many common causes like SPF lookup limit overages or multiple SPF records, external factors like recipient-side DNS caching or transient network problems can also play a significant role, making diagnosis a multi-faceted task.
Key opinions
- Sender DNS Performance: Slow DNS responses and timeouts from the sender's DNS infrastructure can directly cause intermittent SPF validation failures, especially when combined with recipient-side lookup timeouts.
- SPF Record Compliance: Exceeding the 10 DNS lookup limit or the 255-character string limit in an SPF record is a frequent technical cause for intermittent rejections by receiving mail servers.
- Recipient-Side DNS Behaviors: Issues such as recipient mail servers using old DNS resolvers, aggressive SPF lookup timeouts, or persistent DNS caching can lead to intermittent failures, even if the sender's SPF is correctly configured.
- DNS Propagation and Caching: Delays in DNS propagation and the caching of outdated SPF records by various DNS resolvers globally contribute to intermittent delivery problems.
- Reverse DNS Importance: Some receiving mail servers require valid PTR records (reverse DNS) for sending IPs, and a misconfiguration here can lead to intermittent rejections.
- Local DNS Interference: In some scenarios, router or firewall DNS settings within the sender's local network can interfere with proper DNS resolution, causing intermittent outgoing email issues.
Key considerations
- Assess DNS Responsiveness: Proactively monitor your domain's DNS servers for slow responses or timeouts, as these can directly impact SPF validation attempts by recipient servers.
- Optimize SPF Structure: Actively flatten your SPF record to reduce DNS lookup counts below the 10-limit and ensure the record string stays within the 255-character limit to prevent rejections.
- Manage DNS TTLs for Changes: When updating DNS records, temporarily lower the TTL (Time To Live) before making changes and revert it afterward to accelerate propagation and minimize intermittent issues.
- Verify PTR Records: Confirm that all your outbound sending IP addresses have correct and matching PTR records, as some recipient servers check these for anti-spam purposes.
- Utilize DMARC Reporting: Leverage DMARC aggregate reports to gain visibility into SPF validation failures from various sending sources, helping pinpoint problematic configurations.
- Audit SPF for Redundancy: Regularly audit your domain to ensure there is only one SPF TXT record, as multiple records are a common source of intermittent and unpredictable validation results.
- Test Local DNS Configuration: If local network issues are suspected, temporarily bypass your local router or firewall DNS settings to determine if they are interfering with correct DNS resolution for outgoing email.
Marketer view
Email marketer from Email Geeks shares their problem with intermittent email delivery failures to sapo.pt domains, citing SPF-related issues that eventually succeed on retry. They provide their return path domain as e-mail.marktplaats.nl and speculate that sapo.pt might be using an old DNS resolver or mail server, or struggling with the sender's lengthy SPF record, or even issues related to DNS caching or TTL settings on the recipient's side.
24 Jan 2023 - Email Geeks
Marketer view
Email marketer from Email Geeks explains that after checking the sender's return path domain and running diagnostic tools, he found multiple DNS timeouts and slow responses for the sender's DNS. He suggests these intermittent failures, especially when combined with potentially overly aggressive SPF lookup timeouts or an underspecced local DNS resolver at the recipient's domain (sapo.pt), are likely the cause of the delivery issues. He concludes there is nothing the sender can do to fix or mitigate these external issues.
30 May 2023 - Email Geeks
What the experts say
2 expert opinions
Intermittent email delivery problems are often directly attributable to fundamental misconfigurations in a domain's Sender Policy Framework (SPF) and other crucial DNS records, specifically reverse DNS (PTR). These issues surface when an SPF record fails to properly authorize all legitimate sending IP addresses, or if PTR records for outbound mail servers are missing or incorrect. Receiving mail servers, acting as gatekeepers, may then reject or quarantine messages based on these authentication failures, leading to inconsistent and unpredictable delivery outcomes.
Key opinions
- SPF Authorization Gaps: Intermittent delivery failures frequently occur when an SPF record is improperly configured, or when emails originate from an IP address not explicitly listed as authorized, leading to rejections.
- Reverse DNS Deficiencies: Misconfigured or absent PTR (reverse DNS) records for sending IP addresses are a significant cause of intermittent rejections, as many receiving servers rely on these for legitimacy checks.
- Recipient Server Actions: Both SPF and PTR issues can cause receiving mail servers to reject messages outright or route them to the spam or quarantine folders, resulting in the appearance of intermittent delivery.
Key considerations
- Validate SPF Inclusions: Thoroughly review your SPF record to confirm it accurately includes all IP addresses and third-party services authorized to send email on behalf of your domain, preventing rejections for unauthorized sending sources.
- Verify PTR Record Accuracy: Ensure that every IP address used for sending outbound email has a correctly configured reverse DNS (PTR) record that matches your sending domain, as its absence or inaccuracy is a common reason for delivery failures.
- Analyze Bounce Messages: Carefully examine bounce messages and email logs for specific rejection reasons, as they often clearly indicate whether failures are due to SPF validation issues or missing/incorrect PTR records.
Expert view
Expert from Spam Resource explains that intermittent email delivery failures can be caused by misconfigured DNS records, including incorrect reverse DNS, and SPF issues. If an SPF record is improperly set up, or if emails originate from an IP not authorized in the SPF record, receiving servers may reject or quarantine messages.
5 Mar 2024 - Spam Resource
Expert view
Expert from Word to the Wise explains that intermittent email delivery failures can be caused by misconfigured SPF and PTR (reverse DNS) records, as both are crucial for deliverability. If a PTR record is missing or incorrect, or if SPF checks fail due to misconfiguration, email messages may be rejected or quarantined by receiving servers.
20 Oct 2023 - Word to the Wise
What the documentation says
5 technical articles
Intermittent email delivery issues, particularly those linked to SPF and DNS, are often rooted in a combination of factors including DNS caching, excessively complex or incorrect SPF records, and inconsistent DNS server responses. Such failures highlight the critical need for meticulous validation of SPF records, ensuring all authorized sending sources are correctly included, and verifying the proper configuration and global propagation of all essential DNS records like MX, SPF, and DKIM. Utilizing specialized validation tools and understanding DNS propagation times are crucial steps in diagnosing and resolving these unpredictable delivery challenges.
Key findings
- SPF Record Constraints: MXToolbox indicates that intermittent SPF failures often stem from SPF records exceeding the 10 DNS lookup limit or being too long in character count, leading to validation issues.
- DNS Record Inaccuracies: Google Workspace highlights that incorrect or missing DNS records, such as MX, SPF, or DKIM, are frequent causes of intermittent email delivery problems.
- DNS Caching and Propagation: Both Cloudflare and Google explain that DNS caching issues and the time it takes for DNS changes to fully propagate globally are significant contributors to intermittent delivery failures.
- SPF Syntax and IP Inclusion: Microsoft Learn points out that SPF failures can occur if the sending IP address is not explicitly listed in the SPF record or if the record itself contains syntax errors.
- DNS Inconsistency and Timeouts: Cisco notes that intermittent SPF validation failures can arise from inconsistent DNS configurations for the sending domain or if DNS queries time out.
Key considerations
- Utilize SPF and DNS Validation Tools: Employ online tools, such as MXToolbox's SPF lookup, to thoroughly validate your SPF record, ensuring correct syntax and that all included mechanisms resolve without exceeding DNS lookup limits. Likewise, use general DNS lookup tools to verify the configuration and propagation of all essential records.
- Verify Core DNS Records: Confirm that all necessary DNS records, including MX, SPF, and DKIM, are accurately configured and published. These records are fundamental for consistent email delivery.
- Manage DNS Propagation and Caching: Be mindful that DNS changes can take up to 48 hours to fully propagate globally, which can contribute to intermittent issues. Understand DNS caching and Time To Live (TTL) settings to mitigate delays in record updates.
- Ensure Comprehensive SPF Authorization: Explicitly include all authorized sending IP addresses and services within your SPF record to prevent intermittent rejections from sources not listed. Also, validate the SPF record's syntax to avoid errors.
- Maintain DNS Server Responsiveness: Ensure your DNS servers are consistently responsive and that SPF records strictly adhere to RFC specifications. Inconsistent configurations or query timeouts can lead to unpredictable SPF validation failures.
Technical article
Documentation from MXToolbox explains that intermittent SPF failures can stem from DNS caching issues or an SPF record that is too long, exceeding the 10 DNS lookup limit. They recommend using their SPF lookup tool to validate the record and ensure all included mechanisms resolve correctly without excessive lookups.
8 Jan 2023 - MXToolbox
Technical article
Documentation from Google Workspace Admin Help explains that intermittent email delivery failures often relate to incorrect or missing DNS records like MX, SPF, or DKIM. They advise using a DNS lookup tool to verify all necessary records are correctly configured and propagated, noting that DNS changes can take up to 48 hours to fully propagate globally.
9 Oct 2023 - Google Workspace Admin Help
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