Does SPF authenticate the 'Mail-From' address?

Yes, but it's crucial to understand which "From" address it authenticates. SPF (Sender Policy Framework) authenticates the domain found in the email's envelope sender, also known as the 'Mail-From', 'Return-Path', or bounce address. It does not authenticate the 'From' address that you see in your email client.

This is a common point of confusion that has significant implications for email security and DMARC alignment. Let's explore the difference and see how the process works.

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Every email has two 'from' addresses. Understanding the role of each one is the key to understanding how SPF works.

• The Envelope Sender ('Mail-From' or Return-Path): This is a technical address used during the SMTP transaction, specified by the MAIL FROM command. It's not typically visible to the end user. Its primary purpose is to tell the receiving server where to send bounce messages or delivery failure notifications. This is the address that SPF checks.
• The Header From ('From'): This is the address you see in the 'From' field of your email client. It's part of the email's content (the headers) and is intended for the human recipient. Since this address is just text in the email body, it can be easily forged by spammers.

SocketLabs says:

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SPF authenticates the domain used in the “envelope” or return-path email address. This address is used during the transport of the message (from server to server).

The purpose of an SPF record is to create a public list of all the IP addresses that are permitted to send email on behalf of your domain. As described by Kickbox, "SPF is a list of IPs that the domain owner has publicly authorized to send on its behalf." When a receiving mail server gets an email, it performs the following check:

• The receiving server identifies the domain in the 'Mail-From' (Return-Path) address.
• It also notes the IP address of the server that sent the email.
• The server then queries the Domain Name System (DNS) for the SPF record (a TXT record) of the 'Mail-From' domain.
• It compares the sending IP address to the list of authorized IPs found in the SPF record.
• If the sending IP is on the list, the SPF check passes. If it is not, the check fails.

Klaviyo Help Center says:

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SPF allows the receiving mail server to verify that emails coming from a specific domain were sent through an IP address authorized by that domain’s owner.

This distinction is critically important when you implement DMARC. DMARC requires something called 'alignment' to protect your domain from spoofing. For SPF to pass DMARC, the check must not only pass, but the domain used for the SPF check (the 'Mail-From' domain) must also match the domain in the visible 'From' header.

For example, if a spammer sends an email with a From address of ceo@yourcompany.com but uses a Mail-From address of bounces@spammerdomain.com, the SPF check will be performed against spammerdomain.com. Since the spammer controls that domain, they can make the SPF check pass. However, because spammerdomain.com does not align with yourcompany.com, the message will fail DMARC's SPF alignment check.

AutoSPF says:

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In simpler words, SPF alignment means verifying that the sender's IP address is officially authorized to be used for sending emails.

In conclusion, while SPF is a foundational email authentication protocol, it only verifies the authenticity of the hidden 'Mail-From' address. It is the combination of SPF, DKIM, and a DMARC policy that provides robust protection against email spoofing by ensuring the domains a user sees are the domains that have been authenticated.