What steps should be taken when DKIM/SPF fails due to spoofing attempts?
Michael Ko
Co-founder & CEO, Suped
Published 3 Jul 2025
Updated 18 Aug 2025
5 min read
When you notice that emails appearing to be from your domain are failing SPF (Sender Policy Framework) or DKIM (DomainKeys Identified Mail) authentication, especially due to spoofing attempts, it can be a concerning situation. My first reaction is usually to assess the extent of the problem and understand why these failures are occurring. It's not just about the technical failure, but what it means for your domain's reputation and your recipients' trust.
The good news is that if these spoofing attempts are indeed failing SPF or DKIM, it means your email authentication protocols are, to some extent, doing their job. They are detecting that something is amiss. However, simply detecting a failure isn't enough, especially when dealing with malicious spoofing. You need to ensure these failures translate into protective actions that prevent these forged emails from reaching inboxes.
The critical next step involves DMARC (Domain-based Message Authentication, Reporting & Conformance). DMARC builds upon SPF and DKIM by providing instructions to receiving mail servers on how to handle emails that fail authentication and offering reporting mechanisms. It's the policy layer that dictates the action to take when spoofing is detected. My goal is always to make sure those failures lead to the desired outcome: blocking the illegitimate emails.
Diagnosing authentication failures and spoofing attempts
Before you can take effective steps, you need a clear picture of what’s happening. DMARC reports are your best friend here. These reports provide invaluable data on who is sending email on behalf of your domain, whether SPF and DKIM are passing or failing, and which receiving servers are seeing these emails.
Reviewing these reports will help you differentiate between true spoofing attempts and legitimate emails that are simply misconfigured. Sometimes, a legitimate marketing platform or transactional email service might not be properly set up with SPF or DKIM, leading to authentication failures for your own valid messages. This is a common pitfall that I often see.
Pay close attention to the source IP addresses in your DMARC reports. If you see unfamiliar IPs attempting to send mail using your domain, especially from geographic regions you don't operate in, it's a strong indicator of spoofing. These insights are crucial for understanding the scope of the problem and formulating a targeted response.
Analyzing DMARC reports
To effectively diagnose DMARC failures, I recommend regularly reviewing your DMARC aggregate and forensic reports. These reports contain detailed information about email authentication results, including the source IP addresses, sender domains, and authentication outcomes (pass/fail for SPF and DKIM). This process is vital for identifying both legitimate emails that are failing and malicious spoofing attempts. If you're encountering widespread failures, it might be beneficial to review our guide on troubleshooting DMARC failures. Remember, understanding these reports is the first step toward a secure email ecosystem.
Implementing and enforcing DMARC policies
Once you've confirmed that the SPF and DKIM failures are indeed due to spoofing, the most impactful step you can take is to adjust your DMARC policy. The goal is to move from a monitoring-only policy (p=none) to one that instructs receiving mail servers to quarantine or reject emails that fail DMARC authentication. This directly combats spoofing by telling mailbox providers how to handle these illegitimate messages. Cloudflare offers a good overview of how DMARC works to tackle email spoofing.
If you're still at p=none, consider transitioning to p=quarantine. This instructs receiving servers to place failing emails into the recipient's spam or junk folder. Once you're confident that no legitimate emails are being affected, you can then move to p=reject, which instructs servers to outright refuse (bounce) emails that fail DMARC. This is the strongest protection against spoofing and phishing. For more guidance, check out our article on safely transitioning your DMARC policy.
DMARC policy options
p=none: This policy is for monitoring only. It tells receiving servers to do nothing with emails that fail DMARC, but still send you reports. It's ideal for initial setup and ensuring legitimate emails pass authentication without disruption. Refer to our guide to simple DMARC examples for more details on starting with this policy.
p=quarantine: With this policy, emails failing DMARC authentication are sent to the recipient's junk or spam folder. This is a good intermediate step, allowing you to catch most spoofed emails without immediately blocking legitimate ones that might still have configuration issues. Microsoft offers guidance on Microsoft's anti-spoofing protection that can be helpful.
p=reject: This is the strongest policy. Emails that fail DMARC authentication are completely blocked and not delivered. Use this only when you are highly confident that all your legitimate sending sources are fully DMARC compliant. For a deeper dive into this policy, see our article on using DMARC p=reject to combat spoofing.
It’s also important to ensure that all legitimate email sending services you use are properly configured with both SPF and DKIM for your domain. Even if a service sends legitimate emails, if they fail authentication, your DMARC policy could cause them to be quarantined or rejected. This applies to transactional email providers, marketing platforms, and even internal systems. Neglecting this step could lead to legitimate emails landing in spam, even when your domain is being spoofed.
Proactive measures and ongoing maintenance
To prevent future spoofing attempts and maintain a strong email reputation, continuous vigilance is key. This means regular monitoring of your DMARC reports to catch new or evolving spoofing tactics. It also involves periodically reviewing your SPF and DKIM records to ensure they are up-to-date and include all legitimate sending sources.
Another crucial proactive measure is DKIM key rotation. While not strictly necessary for every failure scenario, regularly changing your DKIM keys (e.g., every 6-12 months) adds an extra layer of security. If a key were ever compromised, rotating it would invalidate the old key and prevent its misuse by unauthorized parties. This practice reduces the window of opportunity for attackers. These ongoing efforts are essential for your domain's long-term security against spoofing, phishing, and other malicious activity.
Example DMARC record (for reject policy with reporting)DNS
Actively monitor DMARC aggregate and forensic reports to identify all sending sources for your domain, both legitimate and malicious.
Ensure all legitimate email sending services (marketing platforms, transactional systems) are correctly configured with SPF and DKIM.
Gradually implement stricter DMARC policies (from p=none to p=quarantine, then p=reject) after thorough monitoring.
Regularly rotate your DKIM keys to enhance security against potential compromises.
Educate your team about phishing and spoofing to prevent human error and strengthen your organization's defenses.
Common pitfalls
Failing to monitor DMARC reports, which can lead to legitimate emails being blocked or spoofing attempts going unnoticed.
Moving directly to a p=reject DMARC policy without verifying all legitimate sending sources, causing deliverability issues.
Overlooking third-party senders that send on behalf of your domain, resulting in their emails failing authentication checks.
Ignoring occasional SPF or DKIM failures, assuming they are just 'random glitches' without further investigation.
Not aligning the 'From' header (RFC 5322) with the SPF or DKIM authenticated domain, leading to DMARC failures even with valid records.
Expert tips
Always distinguish between genuine spoofing and legitimate, but misconfigured, email sources by analyzing DMARC data carefully.
Remember that even with perfect email authentication, occasional DNS glitches can cause transient failures, but persistent issues require deeper investigation.
Be aware that email gateways or certain mail providers like Microsoft 365 or Google may sometimes rewrite sender addresses, which can impact SPF or DKIM alignment if not properly managed.
Consider that automated emails, such as those from postmaster addresses, might frequently cause SPF and DKIM failures if not specifically accounted for in your authentication setup.
Implement a robust incident response plan for email security breaches, including steps for domain blacklisting and reputation recovery.
Expert view
Expert from Email Geeks says that DMARC enforcement failures require distinguishing between actual spoofing and legitimate but unauthenticated applications. If an application is legitimate, fix its authentication; if it is spoofing, allow it to fail accordingly.
March 10, 2021 - Email Geeks
Expert view
Expert from Email Geeks states that random DNS glitches can cause occasional legitimate email failures, as perfect deliverability is rarely achievable.
March 10, 2021 - Email Geeks
Protecting your email ecosystem
When DKIM and SPF fail due to spoofing attempts, it’s a clear signal that your domain security is being tested. By actively monitoring DMARC reports, correctly configuring all legitimate sending sources, and progressively implementing stronger DMARC policies, you can significantly enhance your domain's protection. Remember, email security is an ongoing process that requires continuous attention and adaptation to new threats.
