What causes dmarc to fail

Overview

DMARC, or Domain-based Message Authentication, Reporting & Conformance, is an email authentication protocol designed to give email domain owners the ability to protect their domain from unauthorized use, such as email spoofing and phishing. It builds upon two existing mechanisms, SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail), to provide a unified email authentication framework. DMARC allows domain owners to specify policies for how receiving mail servers should handle emails that fail authentication checks, including what to do with them (quarantine, reject, or monitor) and to receive reports about email traffic claiming to be from their domain.

When DMARC checks fail, it means that the email in question could not be verified as legitimate according to the policies set by the domain owner. This can have significant implications, potentially leading to legitimate emails being rejected or quarantined by receiving mail servers, or conversely, allowing fraudulent emails to reach inboxes if the DMARC policy is too permissive or not enforced correctly. Understanding the common causes of DMARC failure is crucial for maintaining email deliverability and protecting brand reputation.

Common Causes of DMARC Failure

1. SPF Record Issues

SPF is a system that authorizes mail servers to send email on behalf of a domain. When a receiving mail server checks an incoming email, it looks up the domain's SPF record in DNS and verifies if the IP address of the sending server is listed as an authorized sender. DMARC failure can occur if:

• Incorrect SPF Record Syntax: Typos, missing mechanisms, or incorrect IP addresses/ranges in the SPF record can cause it to be invalid.
• Too Many DNS Lookups: SPF has a limit of 10 DNS lookups. Exceeding this limit (often due to including too many other SPF records or using complex mechanisms like `include:`) will cause the SPF check to fail.
• Missing or Incorrect SPF Record: If no SPF record exists for the domain, or if it doesn't authorize the sending server's IP address, DMARC will fail.
• SPF Alignment Failure: DMARC requires SPF to align with the 'From' header domain. If the domain in the 'From' header (e.g., `yourcompany.com`) is different from the domain used in the SPF check (the `MAIL FROM` or envelope sender domain), and they don't align according to DMARC rules, the DMARC check will fail.

2. DKIM Record Issues

DKIM provides a way to digitally sign an email with a private key, and the corresponding public key is published in the domain's DNS records. This allows receivers to verify that the email was not altered in transit and originated from the claimed domain. DMARC failure can occur if:

• Invalid DKIM Signature: The DKIM signature attached to the email is malformed, missing, or invalid. This can happen if the email content was modified after signing or if the signing process was flawed.
• Incorrect DKIM Public Key: The public key published in the domain's DNS record (TXT record) is incorrect, or the `selector` used in the DKIM signature doesn't match the selector in the DNS record.
• DKIM Alignment Failure: Similar to SPF, DMARC requires DKIM to align with the 'From' header domain. If the DKIM signature is for a different domain than the one specified in the 'From' header, and they do not align according to DMARC rules, the check fails.
• Missing DKIM Record or Selector: The DKIM TXT record is not published in DNS, or the selector used for signing does not correspond to any published record.

3. DMARC Record Configuration Errors

The DMARC record itself, published as a TXT record in DNS, contains the policy and reporting instructions. Errors in this record can lead to DMARC failure or misinterpretation:

• Syntax Errors: Incorrect formatting, invalid tags (like `p=`, `rua=`, `ruf=`, `sp=`, `adkim=`, `aspf=`), or missing mandatory tags can render the DMARC record unreadable or invalid.
• Incorrect Policy Setting: While not a direct cause of *failure* in the authentication sense, setting the policy to `reject` or `quarantine` when authentication is failing (due to other reasons) will result in emails not reaching the inbox. The policy is what dictates the action taken upon failure.
• Invalid Reporting Addresses: Incorrectly formatted email addresses for `rua` (aggregate reports) or `ruf` (forensic reports) can prevent reports from being delivered, making it harder to diagnose issues.

4. Email Sending Infrastructure and Third-Party Services

Many organizations use third-party services (like marketing platforms, CRM systems, or cloud-based email providers) to send emails on their behalf. If these services are not properly configured to send emails that pass SPF and DKIM checks for your domain, DMARC will fail.

• Unauthenticated Third-Party Senders: If a third-party sender uses its own domain for the `MAIL FROM` address and doesn't have SPF set up correctly for your domain, or if they don't sign emails with DKIM for your domain, DMARC will fail.
• Shared IP Addresses: While not a direct cause of DMARC failure, if a shared IP address used by a bulk sender is blacklisted, it can lead to emails being flagged as spam, and if DMARC is set to `reject`, these emails will be blocked.
• Mismatched Sending Domains: The domain used in the `MAIL FROM` (envelope sender) and the domain used in the DKIM signature must align with the domain in the 'From' header for DMARC to pass. If these are inconsistent due to how a service is configured, DMARC will fail.

5. Email Content and Structure

While less common, certain aspects of email content or structure can indirectly contribute to DMARC failure, especially related to DKIM:

• Modification of Signed Headers: If any headers that are included in the DKIM signature are modified by intermediate servers (e.g., adding `Received` headers in a way that invalidates the signature), the DKIM check can fail, leading to DMARC failure.
• Forwarded Emails: Standard email forwarding mechanisms can sometimes break DKIM signatures or alter the `MAIL FROM` address in a way that causes SPF alignment failures, thus failing DMARC checks. DMARC's `fo` (forwarded override) tag can help manage this, but it requires careful configuration.

Troubleshooting DMARC Failures

Diagnosing DMARC failures involves examining the DMARC reports (`rua` and `ruf`) sent by receiving mail servers. These reports provide detailed information about which authentication mechanisms (SPF, DKIM) passed or failed, and whether alignment occurred. Tools like DMARC analyzers can help parse these reports. It's essential to ensure that both SPF and DKIM are correctly configured and aligned with the 'From' header domain before enforcing a strict DMARC policy (like `reject`). A gradual rollout, starting with a `none` policy for monitoring, then moving to `quarantine`, and finally `reject`, is recommended.