How to interpret DMARC reports for unrecognized email sending sources and low volume DMARC failures?
Matthew Whittaker
Co-founder & CTO, Suped
Published 4 May 2025
Updated 19 Aug 2025
9 min read
Navigating DMARC (Domain-based Message Authentication, Reporting, and Conformance) reports can sometimes feel like sifting through a stack of cryptic documents, especially when you encounter unrecognized email sending sources or a trickle of low-volume DMARC failures. These reports are crucial for understanding who is sending emails from your domain and how recipients are handling them. They provide insights into legitimate email streams and potential unauthorized activity.
The goal of DMARC is to protect your domain from impersonation and phishing by ensuring emails are properly authenticated. When DMARC reports show emails from unfamiliar sources, it can be concerning, raising questions about whether your domain is being spoofed or if there are legitimate, but unconfigured, senders you are unaware of. Similarly, consistent low-volume failures, while seemingly minor, can indicate underlying issues that need attention.
I'll guide you through interpreting these scenarios, distinguishing between genuine threats and common misconfigurations, and outline how to manage your DMARC policy effectively. Understanding these nuances is key to maintaining a healthy email ecosystem and ensuring your messages reach the inbox reliably.
DMARC reports come in two main types: aggregate (RUA) and forensic (RUF). Aggregate reports, typically XML files, provide an overview of all email traffic observed for your domain, including pass/fail rates for SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail), along with the sending IP addresses. Forensic reports, while less common due to privacy concerns, contain more detailed information about individual emails that fail DMARC, often including headers or even full message content. However, most mailbox providers, like major providers, rarely provide these due to the sensitive data they might contain. For most purposes, aggregate reports are your primary tool.
When you open an aggregate report, you'll see various fields that provide a snapshot of your email traffic. Key elements include the reporting organization (e.g., Google, Yahoo), the reporting period, the domain being reported on, and detailed records for each sending IP address. Each IP record specifies the volume of emails, the DMARC disposition (how the email was handled), and whether SPF or DKIM passed or failed, including alignment status. You can learn more about this by reviewing common DMARC tags.
The core challenge lies in connecting the IP addresses and domains listed in the reports back to actual services or senders. An IP address by itself doesn't tell you much, but when combined with the associated DKIM signing domain (the d= tag in the DKIM signature) and the SPF domain, it starts to paint a clearer picture. For example, if you see a Mailchimp DKIM domain like mailchimpapp.net or mcdlv.net with your domain in the Header From field, it suggests a Mailchimp sender.
Report Field
Significance for Identification
Source IP Address
Identifies the server sending the email.
DKIM Domain (d=)
The domain that signed the email, often revealing the ESP (Email Service Provider) or internal system.
SPF Domain (Return-Path)
The domain used for SPF validation, also indicative of the sending service.
Header From Domain
The domain shown to the recipient in the 'From' field of the email client. This is the one DMARC checks for alignment.
DMARC Disposition
Indicates whether the email passed or failed DMARC. Failures can be fail-unaligned (common for legitimate unconfigured senders) or fail.
Identifying unrecognized sending sources
Unrecognized high-volume sending sources are often the most alarming. If your DMARC reports show hundreds of emails from a service like Mailchimp or SendGrid with your Header From domain, but your team claims not to use these services, it points to a few possibilities. One common culprit is a rogue marketing operation within your own organization. Someone might be sending campaigns without proper authorization or knowledge of your email team.
These are not typically spammers, as reputable Email Service Providers (ESPs) like Mailchimp have strict abuse prevention measures, often requiring email address verification before sending. If a rogue sender is using your organizational domain's friendly From, even without proper DKIM alignment, it can still impact your domain reputation, especially if their content is problematic or they link back to your legitimate site.
To address these, you need to initiate an internal investigation. This involves reaching out to various departments, especially marketing, sales, and customer support, to identify if anyone is using an unknown email platform. If you cannot identify the source, increasing your DMARC policy from p=none to p=quarantine or p=reject can force the rogue sender to come forward when their emails start failing deliverability checks. This is a common method for troubleshooting DMARC failures.
Identifying unknown senders
Look at the DKIM d= domain and SPF Return-Path domain. These often reveal the actual email service used, even if the Header From address is your own. For example, zoho.com or smtp2go.com domains in these fields point to those services.
Investigate internal use
Talk to departments that might use third-party services, such as marketing for newsletters, support for ticketing systems (Zoho CRM, for example), or HR for applicant communications. You might uncover a legitimate, but unconfigured, sending source.
Deciphering low volume DMARC failures
Low-volume DMARC failures, often just a few emails per week, are common and can be less alarming than high-volume discrepancies. These often appear as fail-unaligned for SPF or DKIM, even if the raw authentication passes. The key is the DMARC alignment requirement, which mandates that the Header From domain aligns with either the SPF Return-Path domain or the DKIM d= domain. When these don't align, DMARC fails, even if SPF or DKIM technically passed authentication. You can read up on DMARC, SPF, and DKIM for more.
Common causes for low-volume failures include email forwarding, where an email passes through an intermediary server that alters the message in a way that breaks SPF or DKIM alignment. Another source can be poorly configured web forms that spoof the email sender, leading to backscatter. These are often misconfigurations rather than malicious spoofing attempts.
For these background noise failures, it's generally not necessary to panic or rush to an aggressive DMARC policy like p=reject. Instead, carefully analyze the report details. Check if the DKIM d= or SPF Return-Path domains belong to known third-party services that your organization interacts with, even if not for direct email sending. This might indicate an email being generated indirectly by a business tool.
Identifying legitimate senders
Check all domains: Look at the DKIM d= and SPF Return-Path domains for unrecognized sources, even if the Header From is your domain.
Internal audit: Investigate with internal teams (e.g., IT, marketing, sales) if they use any third-party services that might send emails on your behalf, even indirectly.
Look for forwarding: Low-volume failures from large ISPs or services (like Liberty Global Mail Platforms) might indicate email forwarding. This means a legitimate email was forwarded and broke DMARC alignment.
Addressing DMARC failures
Configure legitimate sources: For identified legitimate senders, ensure SPF and DKIM are properly configured for your domain. This involves adding the sender's IPs to your SPF record and setting up custom DKIM signing where possible. You can debug DMARC authentication failures.
Monitor and iterate: Continue to monitor your DMARC reports as you make changes. The goal is to get as close to 100% legitimate email authentication as possible before moving to a stricter DMARC policy. Read about managing DMARC reports.
Progressive policy: Begin with a p=none policy and gradually move to p=quarantine then p=reject. This allows you to gain visibility before enforcing actions. See how to transition your policy.
Actionable steps and policy enforcement
When you're dealing with DMARC failures, especially from unrecognized or low-volume sources, the most critical step is to determine if the sending is legitimate. For high-volume unrecognized sources, like a Mailchimp account that your team denies using, the likelihood of a rogue internal sender is high. This requires internal communication to uncover if a department is using an unapproved email platform.
If a legitimate source is identified, the solution is to properly configure SPF and DKIM for that sender to achieve DMARC alignment. For example, if you use a third-party service, ensure their sending IP addresses are included in your SPF record and that their DKIM signatures align with your Header From domain. This might involve setting up a custom DKIM record for that service.
For low-volume, non-malicious failures, it's generally safe to monitor the situation. However, if the DMARC reports show .nxdomain in the reports, meaning the IP address doesn't map to a domain (no rDNS), these are likely unauthenticated, possibly malicious, sources. While the volume may be low, it is still important to stay vigilant. For a comprehensive understanding of DMARC, it can be useful to consult the official DMARC RFC.
Ultimately, your DMARC policy (the p= tag) dictates how receiving mail servers should treat emails that fail DMARC. Starting with p=none allows you to gather reports without affecting deliverability. Once you've identified and configured all legitimate sending sources, you can gradually move to a more protective policy like p=quarantine (send to spam/junk) or p=reject (block entirely). This progressive approach is crucial to avoid inadvertently blocking your own legitimate emails and ensures a smooth transition to full DMARC enforcement. For more information about this, see our article on simple DMARC examples.
Always start DMARC with a 'p=none' policy to gather data without impacting deliverability.
Regularly review your aggregate DMARC reports to identify all email sending sources.
Collaborate with internal teams to map unknown sending IPs to legitimate services or departments.
Ensure all legitimate third-party senders have proper SPF and DKIM authentication set up and aligned.
Consider a phased approach to DMARC policy enforcement, moving from 'none' to 'quarantine' then 'reject'.
Common pitfalls
Panicking over low-volume DMARC failures without analyzing their root cause, which are often benign.
Ignoring unrecognized high-volume senders, as they could be rogue internal operations or unauthorized use.
Assuming DMARC failures automatically mean malicious spoofing; many are due to misconfigurations or forwarding.
Jumping directly to a 'p=reject' DMARC policy without full visibility, potentially blocking legitimate email.
Relying solely on forensic reports, which are rarely provided by mailbox providers due to privacy concerns.
Expert tips
If Mailchimp emails are failing DMARC alignment, it often indicates a rogue internal user who hasn't configured custom DKIM.
Zoho failures at low volume can often be attributed to email forwarding or misconfigured web forms.
Emails failing with '.nxdomain' (no rDNS) are often from unauthenticated or potentially malicious sources.
Be cautious of marketing platforms using alarmist language like 'threats' for all unauthenticated traffic.
When dealing with suspected rogue internal senders, transitioning to a stricter DMARC policy can force them to self-identify.
Expert view
Expert from Email Geeks says forensic reports are generally not useful and rarely provided by mailbox providers, making them unreliable for troubleshooting.
October 3, 2019 - Email Geeks
Expert view
Expert from Email Geeks says Zoho email traffic with low volumes in DMARC reports is often due to email forwarding or misconfigured web forms on client sites.
October 3, 2019 - Email Geeks
Key takeaways for DMARC report interpretation
Interpreting DMARC reports for unrecognized sending sources and low-volume failures is a critical part of maintaining your email deliverability and protecting your domain from abuse. It requires a blend of technical analysis and internal communication to accurately categorize unexpected email traffic.
The key is to differentiate between genuine spoofing threats, which often involve large volumes from truly unknown or suspicious IPs, and more benign issues like misconfigurations, email forwarding, or rogue internal senders. By systematically investigating each anomaly in your reports, you can ensure all legitimate email streams are properly authenticated and that your DMARC policy effectively protects your domain.
Consistent monitoring and a gradual approach to DMARC policy enforcement will empower you to identify and resolve these issues, enhancing your email security posture and ensuring your messages reliably reach their intended recipients. This continuous refinement is essential for achieving optimal email deliverability in the long run.
Google, 
Yahoo), the reporting period, the domain being reported on, and detailed records for each sending IP address. Each IP record specifies the volume of emails, the DMARC disposition (how the email was handled), and whether SPF or DKIM passed or failed, including alignment status. You can learn more about this by reviewing common DMARC tags.
Mailchimp DKIM domain like mailchimpapp.net or mcdlv.net with your domain in the Header From field, it suggests a Mailchimp sender.
SendGrid with your Header From domain, but your team claims not to use these services, it points to a few possibilities. One common culprit is a rogue marketing operation within your own organization. Someone might be sending campaigns without proper authorization or knowledge of your email team.
Zoho CRM, for example), or HR for applicant communications. You might uncover a legitimate, but unconfigured, sending source.
