What steps should be taken when DKIM/SPF fails due to spoofing attempts?

Summary

When DKIM/SPF fails due to potential spoofing attempts, a multi-faceted approach is necessary. The initial step involves verifying the legitimacy of the email source to determine whether it's a valid application requiring authentication or a malicious spoofing attempt. Analyzing DMARC reports is crucial to pinpoint the failure's origin, distinguishing between a legitimate sender with misconfigured authentication and a malicious actor. Subsequently, enforcing a DMARC policy of 'quarantine' or 'reject' instructs receiving mail servers on how to handle unauthenticated emails, mitigating the impact of spoofing. Proper SPF configuration, including correct syntax and avoiding common errors like exceeding DNS lookup limits or using multiple SPF records, is also vital. In addition, email headers need reviewing to identify the source IP and the domain used for SPF authentication. Finally, continuously monitoring DMARC reports allows ongoing identification and response to spoofing threats.

Key findings

  • Legitimacy Verification: Verify if the email source is legitimate, needing authentication, or a spoofing attempt.
  • DMARC Report Analysis: Analyze DMARC reports to identify the source of SPF/DKIM failures and alignment issues.
  • DMARC Policy Enforcement: Enforce a DMARC policy of 'quarantine' or 'reject' to mitigate the impact of spoofing.
  • SPF Configuration Review: Review and correct SPF records, avoiding common errors like exceeding DNS lookup limits.
  • Header Analysis: Review email headers to identify the source IP address and domain used for SPF authentication.
  • Continuous Monitoring: Continuously monitor DMARC reports to identify and respond to ongoing spoofing threats.

Key considerations

  • Email Gateway Issues: Email gateways like Mimecast/Proofpoint can cause SPF/DKIM failures for automated emails.
  • DKIM Alignment Importance: Lacking DKIM and relying only on SPF alignment can lead to issues with email forwarding and rewriting.
  • Potential DNS Glitches: Random DNS glitches can occasionally cause SPF/DKIM failures.
  • SPF Record Best Practices: Avoid common SPF record errors, like multiple records or incorrect syntax, by validating the DNS records.
  • RFC 7208 Adherence: SPF record syntax should adhere to RFC 7208.
  • Hard Fail vs. Soft Fail: Weigh the implications of a Hard Fail vs. Soft Fail SPF policy. Hard Fail will reject non-authenticated emails, but take caution to not reject wanted emails.
  • SPF Record Limitations: Be aware of the SPF record lookup limit of 10 DNS lookups when configuring SPF.
  • Authentication Validation: For legitimate sources experiencing SPF/DKIM failures, prioritize fixing the authentication issues, by ensuring their IP addresses are in the SPF Record.

What email marketers say
8Marketer opinions

When DKIM/SPF fails due to spoofing attempts, the primary steps involve investigation, mitigation, and continuous monitoring. Initial actions include analyzing DMARC reports to determine the source of the failure, distinguishing between legitimate sending sources requiring authentication and actual spoofing attempts. Implementing or adjusting DMARC policies to 'quarantine' or 'reject' is crucial for handling unauthenticated emails. Reviewing and correcting SPF records, ensuring proper syntax and avoiding common errors like exceeding DNS lookup limits or using multiple SPF records, is essential. Employing online tools to validate SPF configurations and monitoring DMARC reports continuously helps in identifying and responding to ongoing spoofing threats.

Key opinions

  • DMARC Analysis: Analyzing DMARC reports is critical to identify the source and nature of SPF/DKIM failures.
  • DMARC Policy: Implementing a 'quarantine' or 'reject' DMARC policy is crucial for mitigating spoofing impact.
  • SPF Configuration: Proper SPF record configuration, avoiding common errors like exceeding DNS lookup limits, is vital.
  • Continuous Monitoring: Continuous monitoring of DMARC reports is essential to identify and respond to ongoing spoofing attempts.

Key considerations

  • Email Gateways: Email gateways can sometimes cause SPF/DKIM failures due to automated emails.
  • DKIM Alignment: Lacking DKIM with only SPF alignment can lead to issues with email forwarding and rewriting.
  • DNS Issues: DNS glitches can occasionally cause SPF/DKIM failures.
  • SPF Record Errors: Avoid common SPF record errors, such as multiple records or incorrect syntax, by validating the DNS records.
  • Soft vs Hard Fail: Use SPF Hard Fail (-all) to instruct receiving servers to reject unauthenticated mail, but be cautious of legitimate mail sources failing.
Marketer view

Email marketer from EasyDMARC shares that implement DMARC with a policy of 'quarantine' or 'reject' to instruct receiving mail servers on how to handle emails that fail SPF and DKIM checks. This helps prevent spoofed emails from reaching the inbox.

December 2024 - EasyDMARC
Marketer view

Email marketer from URIports explains to continuously monitor DMARC reports to identify and respond to spoofing attempts. These reports provide valuable information about the source of failing emails and allow you to adjust your SPF and DKIM settings accordingly.

September 2023 - URIports
Marketer view

Email marketer from Email Hippo explains the difference between a Soft Fail (~all) and a Hard Fail (-all). The Hard Fail will tell receiving servers to reject messages that do not match your SPF record. Soft fail is a more lenient setting to allow for mail to still make it through, but is not recommended.

July 2023 - Email Hippo
Marketer view

Marketer from Email Geeks shares options for debugging DMARC failures: (1) Email Gateways like Mimecast/Proofpoint may cause SPF & DKIM failures for automated emails. (2) Lacking DKIM with only SPF alignment can cause issues with Office365 and Google Calendar rewriting MailFrom. (3) DNS glitches or ESP infrastructure issues can cause hard-to-identify problems.

October 2024 - Email Geeks
Marketer view

Email marketer from MXToolbox shares to avoid common SPF record errors such as exceeding the 10 DNS lookup limit, using multiple SPF records, and incorrect syntax. Use a tool such as MXToolbox to validate your DNS records.

May 2023 - MXToolbox
Marketer view

Email marketer from Stackoverflow states SPF failures can happen because of multiple SPF records. Make sure you only have one SPF record defined. This is a common reason for SPF failures.

December 2022 - Stackoverflow
Marketer view

Email marketer from Mailhardener explains that when DKIM/SPF fails, first investigate the source of the failure using DMARC reports. Check if it's a legitimate sending source that needs to be authenticated, or if it's a spoofing attempt. If it's spoofing, ensure your DMARC policy is set to 'reject' or 'quarantine' to minimize impact.

May 2021 - Mailhardener
Marketer view

Email marketer from Reddit shares to review your SPF records and ensure they're correctly configured. Often, issues arise from exceeding the DNS lookup limit or incorrect syntax. Use online SPF record testing tools to validate your configuration.

January 2025 - Reddit

What the experts say
4Expert opinions

When DKIM/SPF fails due to potential spoofing, initial steps involve verifying the legitimacy of the source, distinguishing between a rogue application needing authentication and actual spoofing attempts. Analysis of failure reports is crucial to pinpoint the source, identifying if it's a legitimate sender with misconfigured authentication or a malicious actor. Enforcing a DMARC policy of 'quarantine' or 'reject' is then key to instruct receiving servers on handling unauthenticated emails, thereby mitigating the impact of spoofing. Random DNS glitches should also be considered as a potential cause of occasional failures.

Key opinions

  • Legitimacy Check: Verify if the source is a legitimate application requiring authentication or an actual spoofing attempt.
  • Failure Report Analysis: Analyze failure reports to identify the source of the SPF/DKIM failure.
  • DMARC Policy Enforcement: Enforce a DMARC policy of 'quarantine' or 'reject' to mitigate spoofing.

Key considerations

  • DNS Glitches: Random DNS glitches can occasionally cause SPF/DKIM failures.
  • Authentication Fix: If the source is legitimate, focus on fixing the authentication issues.
Expert view

Expert from Email Geeks explains to make sure the spoofing is legitimate and not a rogue application needing authentication. If legitimate, fix authentication; otherwise, let it fail.

September 2021 - Email Geeks
Expert view

Expert from Word to the Wise shares that when spoofing attempts are detected via SPF/DKIM failures, enforcing a DMARC policy of 'quarantine' or 'reject' is crucial. This instructs receiving mail servers to handle unauthenticated emails, mitigating the impact of spoofing.

March 2022 - Word to the Wise
Expert view

Expert from Spamresource explains that to address SPF and DKIM failures, you need to start by analyzing the failure reports. These reports can help pinpoint the source of the failure, whether it's a legitimate sender with misconfigured authentication or a malicious actor attempting to spoof your domain.

November 2024 - Spamresource
Expert view

Expert from Email Geeks responds that random DNS glitches can cause occasional email failures; achieving 99.99% reliability is often considered excellent.

March 2023 - Email Geeks

What the documentation says
5Technical articles

When DKIM/SPF fails due to suspected spoofing, several steps should be taken. Firstly, analyze email headers to identify the source IP and domain used for SPF authentication and verify if the sending server is authorized. Secondly, ensure the SPF record syntax is correct, properly published in DNS, and includes authorized sending servers using mechanisms like 'include', 'ip4', and 'ip6'. Be mindful of the SPF record lookup limit of 10 DNS lookups. Analyzing DMARC failure reports provides insights into spoofing attempts and SPF/DKIM alignment issues. Update DNS SPF records following RFC 7208 syntax, including all legitimate sending sources using 'ip4:' and 'ip6:'.

Key findings

  • Header Analysis: Review email headers to identify the source IP and domain for SPF authentication.
  • SPF Record Syntax: Ensure SPF record syntax is correct and properly published in DNS.
  • Authorized Servers: Confirm authorized sending servers are included in the SPF record using mechanisms like 'include', 'ip4', and 'ip6'.
  • DMARC Reports: Analyze DMARC failure reports to identify spoofing attempts and alignment issues.

Key considerations

  • SPF Lookup Limit: Be aware of the SPF record lookup limit of 10 DNS lookups.
  • RFC 7208: SPF record syntax should adhere to RFC 7208.
  • Hard Fail: Update SPF records to include all legitimate sending sources to prevent SPF Hard Fail errors.
Technical article

Documentation from RFC Editor explains that SPF record syntax should follow RFC 7208 and include mechanisms like 'a', 'mx', 'ip4', 'ip6', 'include', and 'all' to define authorized sending sources. Understanding the syntax is key to configuring SPF correctly.

December 2022 - RFC Editor
Technical article

Documentation from Microsoft Learn explains that to troubleshoot SPF failures, check the SPF record syntax and ensure it's correctly published in DNS. Confirm the sending server is listed in the SPF record using 'include:' or 'ip4:'/'ip6:' mechanisms. Also, be aware of SPF record lookup limits (10 DNS lookups) which can cause SPF to fail.

August 2021 - Microsoft Learn
Technical article

Documentation from DMARC.org explains that analyzing DMARC failure reports (both aggregate and forensic) can reveal spoofing attempts and SPF/DKIM alignment issues. These reports provide insights into the source of unauthorized email and help identify domains that are being spoofed.

May 2023 - DMARC.org
Technical article

Documentation from AuthSMTP explains that to fix SPF Hard Fail errors, update your DNS SPF record to ensure it includes all legitimate sending sources. The common syntax to use would be ip4: and ip6:

April 2022 - AuthSMTP
Technical article

Documentation from Google Workspace Admin Help explains that if SPF fails, review the email headers to identify the source IP address and the domain used for SPF authentication. Verify that the sending server is authorized to send emails on behalf of your domain and ensure your SPF record includes the sending server's IP address or domain.

September 2022 - Google Workspace Admin Help