DMARC Failures and What To Do About Them

DMARC (Domain-based Message Authentication, Reporting & Conformance) is a security protocol that helps prevent email spoofing and phishing.

In Simple Terms:

• It tells email providers how to handle emails that fail authentication checks (SPF & DKIM).
• It helps protect your domain from being used in spam or phishing attacks.
• It provides reports so you can see who is sending emails using your domain.

How It Works:

1. SPF Check – Verifies if the email is sent from an authorized server.
2. DKIM Check – Confirms the email hasn’t been tampered with.
3. DMARC Policy – If SPF & DKIM fail, the receiving email provider follows your DMARC rule:
   • None → Let the email through (just monitor).
   • Quarantine → Mark it as spam.
   • Reject → Block it entirely.

Why It Matters:

• Stops spammers from faking emails from your domain.
• Improves email deliverability and trust.
• Gives you visibility into email activity through DMARC reports.

A small set of DMARC failures in a large mailing campaign can happen for several reasons. Here are some common causes:

1. Forwarding Without ARC (Authenticated Received Chain)

• If your emails are forwarded by intermediate servers that do not use ARC, the original SPF and DKIM authentication may break, causing DMARC failures.
• Some recipients might be using email providers that modify the headers or content slightly, invalidating DKIM signatures.

2. SPF Alignment Issues

• If the email is sent through a third-party email service (e.g., Mailchimp, SendGrid), the return-path domain might not match your From domain, leading to SPF misalignment.
• Ensure the email service provider is authorized via SPF records (include:thirdparty.com).

3. DKIM Signature Breakage

• Some email providers may modify parts of the message (e.g., subject line encoding, extra headers) after sending, which can break DKIM validation.
• Ensure your DKIM signature uses a relaxed canonicalization setting (c=relaxed/relaxed).

4. Mailing List Modifications

• If your email passes through a mailing list (Google Groups, Listserv, etc.), the list might modify the email (adding a footer, subject tags, etc.), breaking DKIM.
• Some mailing lists rewrite the From address (to something like via list@example.com) to prevent DMARC failures, but not all do.

5. DMARC Policy at Recipient Side

• Some receiving email providers might have stricter DMARC evaluation, rejecting emails that would otherwise pass at other providers.
• This can cause a few failures in a large campaign if some ISPs enforce DMARC more strictly than others.

6. Temporary DNS Issues

• If the recipient’s mail server queries your SPF/DKIM/DMARC records during an outage or delay, it may fail verification.

7. Spoofing or Interference

• Some failures could be due to unauthorized parties attempting to spoof your domain.
• Review the failure reports carefully to ensure they originate from your legitimate mail servers.

How to Troubleshoot?

• Check DMARC reports to identify which ISPs and recipients are failing.
• Use tools like dmarcian, MXToolbox, or Postmark to analyze your domain’s authentication setup.
• Ensure you’re using both SPF and DKIM with proper alignment.
• Consider enabling ARC support if forwarding is a major factor.