What are SPF, DKIM, and DMARC, and when are they needed?
Matthew Whittaker
Co-founder & CTO, Suped
Published 27 Apr 2025
Updated 18 Aug 2025
7 min read
Email is a cornerstone of modern communication, but its open nature makes it vulnerable to abuse like spoofing and phishing. To combat these threats and ensure legitimate emails reach their intended recipients, three foundational email authentication protocols were developed: SPF, DKIM, and DMARC. These acronyms represent critical mechanisms that verify sender identity and message integrity, significantly impacting your email deliverability and domain reputation.
Understanding these protocols is no longer optional, especially with major mailbox providers like Gmail and Yahoo tightening their requirements. Without proper implementation, your emails risk being flagged as spam, rejected, or even blocked, hindering your communication efforts. We'll break down what each protocol does and why their combined use is essential for robust email security.
Sender Policy Framework (SPF) is the oldest of the three authentication standards. It allows a domain owner to specify which mail servers are authorized to send email on behalf of their domain. This is done by publishing an SPF record in their Domain Name System (DNS) as a TXT record. When an email arrives, the recipient's mail server checks the sending IP address against the SPF record published by the sender's domain. If the IP is not listed, the email may be treated as suspicious.
The primary purpose of SPF is to prevent email spoofing, where malicious actors send emails pretending to be from your domain. Without an SPF record, it's easier for spammers to use your domain for illicit activities, potentially leading to your domain being placed on an email blacklist or blocklist. It's crucial to include all legitimate sending sources in your SPF record, from your primary mail server to any third-party email service providers (ESPs).
However, SPF has limitations. It only checks the Return-Path (or Envelope-From) address, not the visible From address that users see. This is where DKIM and DMARC come into play to provide more comprehensive protection. You also need to be careful not to exceed the 10 DNS lookup limit for SPF records, as this can lead to validation failures.
DomainKeys Identified Mail (DKIM) adds a digital signature to your outgoing emails. This signature is generated using a private key kept by the sender and can be verified by recipients using a public key published in your domain's DNS. Think of it as a tamper-proof seal on your email, ensuring that the message hasn't been altered in transit and that it genuinely originated from your domain.
When a recipient's mail server receives an email, it looks for the DKIM signature in the email headers. It then retrieves the public key from the sender's DNS record using a specific DKIM selector. If the signature matches, the email is considered authentic. This process helps prevent phishing attacks and ensures the integrity of your email content, improving trust with recipients.
Unlike SPF, DKIM verifies the message content and headers, providing an additional layer of security. Even if an email is forwarded, DKIM signatures typically remain intact, which SPF alone cannot guarantee. Proper DKIM setup is a crucial step in avoiding the spam folder and establishing your domain's credibility.
What is DMARC?
Domain-based Message Authentication, Reporting, and Conformance (DMARC) builds upon SPF and DKIM by providing a framework for domain owners to instruct receiving mail servers on how to handle emails that fail authentication checks. It also offers a reporting mechanism, allowing senders to receive feedback on how their emails are being handled globally.
DMARC introduces the concept of alignment, meaning that the domain in the From header (what the user sees) must match the domain used for SPF and/or DKIM authentication. This is critical for preventing direct domain spoofing. A DMARC record, also a DNS TXT record, specifies a policy for failed emails: none (monitor only), quarantine (send to spam), or reject (block entirely).
The reporting feature of DMARC is invaluable, providing XML reports (aggregate and forensic) that detail email authentication results. These reports help identify unauthorized sending sources and troubleshoot legitimate email flows that might be failing authentication. Setting up DMARC, particularly moving beyond a p=none policy, requires careful monitoring but offers significant benefits for email security.
Implementing a DMARC policy
When deploying DMARC, it's best to start with a p=none policy. This allows you to gather reports and monitor your email traffic without impacting deliverability. Once you've analyzed the reports and are confident all legitimate mail is authenticating correctly, you can gradually move to p=quarantine and eventually p=reject. This phased approach minimizes the risk of inadvertently blocking legitimate emails.
While each of these protocols offers a layer of protection individually, their real power comes from being implemented together. SPF and DKIM verify sender identity and message integrity, but DMARC acts as the conductor, orchestrating how recipient servers should react to emails that fail these checks and providing valuable insights back to the sender. This layered approach creates a robust defense against email fraud.
For comprehensive email security and deliverability, you need all three. Without SPF, your domain is vulnerable to direct spoofing by unauthorized senders. Without DKIM, messages can be tampered with in transit without detection. And without DMARC, you lose the ability to instruct mailbox providers on how to handle unauthenticated mail, giving them free rein and providing you with no feedback on why your mail might be failing authentication.
The combination of SPF, DKIM, and DMARC is not just about security, it's about trust. Mailbox providers increasingly rely on these authentication standards to assess a sender's legitimacy. Properly configured records signal to these providers that you are a responsible sender, leading to better inbox placement and improved email deliverability. This is especially true with recent shifts in requirements by major email providers like Google and Yahoo.
Protocol
Primary function
Benefit
SPF
Authorizes sending IPs for a domain
Prevents unauthorized senders from using your domain
DKIM
Digitally signs outgoing emails
Ensures message integrity and origin authenticity
DMARC
Establishes policy for unauthenticated mail; provides reporting
Controls delivery of fraudulent emails; offers visibility into abuse
The necessity of email authentication
Implementing SPF, DKIM, and DMARC is a critical step for anyone sending emails from a custom domain. Whether you're a small business, a large enterprise, or an individual sender, these protocols protect your brand reputation, prevent your emails from being misused by bad actors, and significantly improve your chances of reaching the inbox. They are fundamental pillars of modern email security and deliverability.
Ignoring these authentication methods can lead to severe consequences, including reduced email deliverability, loss of customer trust, and even damage to your brand's credibility. Investing the time and resources to properly configure these records is a non-negotiable step for anyone serious about their email strategy.
Views from the trenches
Best practices
Align your SPF and DKIM authentication with your visible "From" domain for optimal DMARC compliance and improved deliverability.
Start with a DMARC policy of p=none to monitor your email traffic and gather reports without risking legitimate email delivery.
Regularly review your DMARC reports to identify any unauthorized sending sources or legitimate email streams failing authentication.
Ensure all services sending email on behalf of your domain (ESPs, transactional email services) are correctly configured for SPF and DKIM.
Keep your DNS records clean and up to date, removing any defunct SPF or DKIM entries to avoid validation issues.
Common pitfalls
Exceeding the 10 DNS lookup limit for SPF records, which can cause SPF authentication failures.
Failing to align the "From" header domain with the SPF or DKIM authenticated domains, leading to DMARC failures.
Jumping straight to a p=reject DMARC policy without proper monitoring, resulting in legitimate emails being blocked.
Not configuring SPF and DKIM for all sending sources, leaving gaps in your domain's authentication.
Ignoring DMARC reports, thus missing critical insights into email abuse and deliverability issues.
Expert tips
Consider that DMARC is primarily for enforcement and reporting based on SPF and DKIM. SPF defines authorized senders, and DKIM provides a tamper-proof signature.
Remember that DMARC is the only protocol that explicitly links the authentication results back to the visible "From" address that end-users see, which is key for anti-spoofing.
While SPF and DKIM alignment with the visible "From" address isn't strictly required by their specifications, it is a best practice for better deliverability, even before DMARC is considered.
It's true that setting up DMARC correctly can be complex and might initially cause deliverability issues if not carefully managed. It's not always a must-have for every situation, especially for smaller senders without significant brand impersonation risk, but it's becoming increasingly necessary.
If you're considering implementing BIMI (Brand Indicators for Message Identification), a DMARC policy of p=quarantine or p=reject is a prerequisite.
Marketer view
Marketer from Email Geeks says SPF signifies which IP addresses are authorized to send mail for your domain.
October 1, 2023 - Email Geeks
Marketer view
Marketer from Email Geeks says DKIM indicates that a message was digitally signed, confirming its authorization by the sender.
