Email forwarding and the involvement of third-party security services' outbound SMTP servers can significantly impact DMARC authentication in several ways. Understanding why this happens requires a grasp of how DMARC and email forwarding work, especially in the context of modern email security practices.

Email Forwarding and DMARC

When an email is forwarded, the original From header, which indicates the sender's email address, typically remains unchanged. DMARC authentication checks both SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail) against this From domain. Here's where forwarding complicates things:

  1. SPF Failures: SPF checks if the email originates from an IP address authorized by the sender's domain DNS records. When an email is forwarded, the forwarding server's IP address is what the receiving server sees, not the original sender's IP. Since the forwarding server is unlikely to be listed in the sender's SPF record, SPF fails.
  2. DKIM Failures: DKIM involves a digital signature added to the email header by the sender. Forwarding servers might modify the email content (even slightly, such as changing the email format or adding a footer), which can invalidate the DKIM signature because the email content no longer matches the signature.

Involvement of Third-Party Security Services

Third-party email security services often act as intermediary SMTP servers that scan emails for spam, phishing, malware, and other security threats. These services might also perform actions like archiving, encryption, or even content modification for security purposes (e.g., URL rewriting for anti-phishing measures).

  1. Outbound SMTP Servers: These services use their SMTP servers to send (or resend) emails after processing. If an email is forwarded through such a service (either because the service is used by an intermediary like a mailing list or by the recipient's organization directly), the email's SPF alignment might fail. This is because the service's SMTP server is not included in the sender's SPF record.
  2. Why It Affects DMARC: When these security services forward or process emails, they might change the email in ways that affect DKIM or cause SPF checks to fail upon the next hop. Since DMARC requires either SPF or DKIM (with proper alignment) to pass, alterations by these services can lead to DMARC failures.

Solutions and Mitigations

  1. Encourage Direct SMTP Delivery: Where possible, encourage the use of direct SMTP delivery mechanisms that bypass traditional forwarding, thus avoiding SPF and DKIM issues.
  2. Use of ARC: The Authenticated Received Chain (ARC) protocol can help by allowing intermediaries (like forwarding services) to sign messages, preserving authentication results even when traditional SPF and DKIM checks would fail.
  3. Communication with Third-Party Services: Engage with your third-party email security providers to understand how they handle forwarded emails and if they offer solutions or best practices to minimize DMARC failures.

Understanding the impact of email forwarding and third-party security services on DMARC is crucial for maintaining email deliverability and security. Implementing best practices and protocols like ARC, alongside effective communication and configuration strategies, can help mitigate these issues.