DMARC alignment ensures a relationship between the "From" address in the email header and the domains used in SPF (via the "Return-Path") and DKIM (via the "d=" tag). Before diving deeper into DMARC (Domain-based Message Authentication, Reporting, and Conformance) alignment, it’s essential to understand how the underlying protocols work:
- SPF (Sender Policy Framework): Verifies if the IP address sending the email is authorized to do so on behalf of the domain. However, SPF validates only the Return-Path address and not the visible "From" address, which can still be spoofed.
- DKIM (DomainKeys Identified Mail): Adds a digital signature to the email, ensuring it hasn’t been altered during transit. While DKIM verifies email integrity, it doesn’t confirm which domain actually signed the email.
DMARC bridges these gaps by requiring alignment between the "From" address and the domains validated by SPF and DKIM, thereby improving email authentication and reducing the risk of spoofing.
There are two types of DMARC alignment: Relaxed and Strict.
DMARC alignment can be strict or relaxed, based on your SPF (aspf) and DKIM (adkim) settings. By default, DMARC uses relaxed alignment, allowing the "From" address and the SPF/DKIM domain to be from different subdomains of the same organization. This is ideal for domains with multiple subdomains. Strict alignment requires an exact match between all domains, providing more security but may block legitimate emails from subdomains.
Relaxed Alignment
aspf=r: The domain in the Return-Path (SPF check) only needs to match the organizational domain of the “From” address. For instance, emails sent from mail.example.com would align with example.com.
adkim=r: The domain in the DKIM signature (d=) must match the organizational domain of the "From" address, allowing for subdomains.
Strict Alignment
aspf=s: The Return-Path domain must exactly match the “From” domain. For example, emails from mail.example.com will not align if the “From” domain is example.com.
adkim=s: The domain in the DKIM signature must exactly match the "From" domain, without allowing subdomains.
For domains with subdomains or third-party email services, relaxed alignment provides smoother operations without sacrificing authentication. Strict alignment offers enhanced security and is best for simpler domain setups where exact matches are possible. The choice between these settings depends on your security requirements and email infrastructure complexity. If your DMARC policy is set to p=reject, mail servers will block suspicious emails, even with relaxed alignment.
How Did DMARC Improve SPF and DKIM Protocols?
While SPF and DKIM were already effective at verifying the authenticity of sending sources, they had certain limitations that DMARC addressed:
1. Links Authentication to the "From" Header
SPF and DKIM verify technical headers (e.g., "Return-Path" for SPF or "DKIM-Signature"), but attackers can still spoof the "From" header visible to users. DMARC ensures alignment between the "From" header and the SPF/DKIM-validated domain, making it more difficult to spoof the sender's identity.
2. Offers Clear Policy Control
DMARC allows domain owners to define how receiving servers should treat emails that fail SPF/DKIM checks (e.g., none, quarantine, or reject). Without DMARC, SPF/DKIM failures might not lead to actionable outcomes.
3. Includes Reporting Features
DMARC provides reporting tools (aggregate and forensic reports) that give domain owners insights into how their domains are being used or misused—something SPF/DKIM alone do not offer.
4. Enhances Deliverability
By ensuring that legitimate emails are properly aligned with SPF and DKIM, DMARC helps receiving servers identify legitimate messages and filter out fraudulent ones, reducing false positives and improving deliverability.
For a more detailed explanation of DMARC alignment, you can refer to this article.
To summarize, DMARC alignment is a key element of email security, addressing gaps that SPF and DKIM protocols may leave unprotected. By ensuring alignment between both protocols and the visible "From:" address, organizations can safeguard their domains against spoofing attacks while ensuring that legitimate emails reach their intended recipients.