SPF
In the context of DMARC, SPF (Sender Policy Framework) is one of the two primary authentication methods used to verify if an email is legitimate. However, DMARC doesn’t just accept a simple SPF pass; it requires something more specific and secure called “alignment.”
Let’s break it down.
1. What is SPF? (The Foundation)
Section titled “1. What is SPF? (The Foundation)”First, it’s important to understand what SPF does on its own.
Think of SPF as a public “guest list” for your domain’s email. You publish this list as a TXT record in your DNS. The list contains all the IP addresses of the mail servers that you have authorized to send emails on behalf of your domain (yourcompany.com).
When an email server receives a message, it looks at the sender’s IP address and checks your SPF record. If the IP address is on the list, the email passes the SPF check. If it’s not, it fails.
Purpose: SPF’s primary job is to prevent basic domain spoofing by verifying that the sending server is an authorized machine.
2. How DMARC Uses SPF
Section titled “2. How DMARC Uses SPF”DMARC acts as an overarching policy layer that uses the results of SPF (and/or DKIM) to make a final decision. For DMARC, a simple SPF “pass” is not enough.
DMARC asks two questions regarding SPF:
- Did the email pass the standard SPF check?
- Is the SPF result “aligned” with the “From” address?
3. The Crucial Concept: SPF Alignment
Section titled “3. The Crucial Concept: SPF Alignment”This is the most important part of SPF’s role in DMARC.
SPF alignment means that the domain in the visible “From” address (the one your recipient sees, e.g., yourcompany.com) must match the domain that was authenticated during the SPF check.
Why is this so important? Without alignment, a scammer could send an email from their own server (scammer.net) which has a perfectly valid SPF record, but put your company’s address ([email protected]) in the “From” field. A basic SPF check would pass because it’s validating scammer.net, but the email is pretending to be from you. Alignment closes this loophole.
Summary
Section titled “Summary”In the DMARC ecosystem, SPF’s role is to provide IP-based sender verification. It answers the question, “Did this email come from an approved server?”
However, for that answer to be meaningful to DMARC, it must be aligned. The approved server must be authorized specifically for the domain that the email claims to be from. Without alignment, an SPF pass is ignored by DMARC, and the email will need to rely on DKIM alignment to pass the overall DMARC check.