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:
SPF Check – Verifies if the email is sent from an authorized server.
DKIM Check – Confirms the email hasn’t been tampered with.
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.
