DKIM vs. SPF: Do I Need Them Both?

5 min readMar 24, 2021

By Brent Sleeper, senior manager of product marketing, Agari

This article originally appeared on the Email Security Blog.

Which should you use: DKIM, SPF, or both? We’re going to cover these terms, when you should use them, what they do — and how best to protect your email domains.

Is it Either/Or — or Both?

Is it necessary to use both SPF and DKIM? While not mandatory, it’s highly recommended to use both SPF and DKIM to protect your email domains from spoofing attacks and fraud while also increasing your email deliverability.

At a time when millions of corporate employees remain working from home due to COVID-19 measures, email has never been more important. Or more profitable for email crime networks. Today, fraudsters of all stripes spoof corporate email domains in phishing attacks and business email compromise (BEC) scams that fuel nearly $9 billion in business losses each year.

Get impersonated in these attacks, and your company could face lost business and significant reputational damage in the marketplace. Even if you avoid regulatory fines or lawsuits, your own legitimate email marketing and communications programs can see deliverability rates tank — if your domains aren’t blacklisted altogether.

So what is SPF (Sender Policy Framework)? And what is DKIM (DomainKeys Identified Email)? They both are important email security standards designed to help prevent hackers from spoofing your domains for use in email attacks targeting your customers, partners, and the general public. To understand why, let’s take a look at these standards, why it’s smarter to leverage both instead of just one — and how to do it right.

How Domain Spoofing Works

In order to spoof an email, all a fraudster has to do is set up or compromise an SMTP server. From there, they can manipulate the ‘From’, ‘Reply-To’, and ‘Return-Path’ email addresses to make their phishing emails appear to be legitimate messages from the individual or brand they’re impersonating.

This identity deception is made possible by the fact that SMTP — the Simple Message Transfer Protocol used by email systems to send, receive, or relay outgoing emails — lacks a mechanism for authenticating email addresses.

Early email authentication standards such as S/MIME failed to gain enough traction to make much of a dent against this threat. But beginning in the mid-2000s, a pair of emerging email security standards started to succeed where other approaches failed — SPF and DKIM.

How SPF Works

At its most essential, SPF allows email senders to specify which IP addresses are allowed to send email from a given domain. For example, a domain owner can stipulate that only IP is allowed to send email from by publishing that policy as a TXT record in the specified domain’s DNS. You can see which servers are authorized to send emails for your domains by using a tool to look up SPF records.

During an SPF check, receiving email servers query the DNS records associated with your sending domain to verify that the IP address used to send the email is listed in the SPF record. If it isn’t, the email will fail authentication — helping to weed out malicious emails attempting to exploit the associated domain.

How DKIM Works

DKIM uses asymmetric encryption to give email senders a way to digitally sign all the outgoing email from a given domain, and publish the public key(s) necessary to validate those digital signatures. This enables receiving email providers to confirm that no changes have been made to the email in transit. Learn more with our DKIM setup guide. Once you do, use a tool to look up DKIM Records to make sure receiving email servers can locate your public key.

When an SMTP server receives an email with such a signature in the header, the server asks the sending domain’s DNS for the public key TXT record. Using the public key, the receiving server is able to verify whether the email was actually sent from that domain.

If the check fails, or if the signature doesn’t exist, the receiving email service provider might mark the email as spam or block the sender’s IP address altogether. This makes it harder for fraudsters to make emails look like they came from your domain address.

Which One is Better?

Ultimately, this isn’t an either/or proposition — it’s a “better together” scenario. That’s because SPF and DKIM address two integral, but separate, issues central to email security.

SPF helps confirm whether an email purporting to come from your company was in fact sent from one of your established IP addresses. And DKIM confirms that the email hasn’t been faked or altered on its way to the intended recipient.

But it’s also important to note that whether they’re used on their own or together, DKIM and SPF do not provide a complete solution for email authentication. For that, we’ll need to add an additional acronym to the conversation: DMARC.

Why DMARC Makes All the Difference

First introduced in 2012, Domain-based Message Authentication, Reporting & Conformance (DMARC) is a standard email authentication protocol that adds a policy layer to SPF and DKIM.

With DMARC, companies can publish policies telling email providers when they should rely on DKIM and SPF for a given domain, and what to do when messages fail either of those tests. DMARC’s most aggressive enforcement policy option is reject (p=reject), which means email messages that don’t pass DMARC authentication will be rejected by the receiving email server and stopped from being delivered to the intended recipient. To learn more, read our DMARC setup guide.

It’s relatively easy to generate a DMARC record and assign it to a DNS. But for large organizations, implementing DMARC across a large number of domains can get very complicated, very fast. Using email ecosystem management solutions designed to help organizations make full use of SPF, DKIM, and DMARC authentication can dramatically simplify the process — causing phishing emails sent by fraudsters seeking to impersonate their businesses to drop to near zero in a matter of weeks.

What’s more, organizations that do adopt these solutions and approaches have realized extraordinary results. According to Forrester Research, organizations using Agari Brand Protection, for example, have seen email conversion rates climb an average 10%, leading to an average $4 million boost in revenues thanks to increased email engagement. Factor in other costs associated with brand impersonation, including finding and shutting down phishing sites, and Forrester reports organizations can see an average 326% ROI.

Plus, using newer, DMARC-enabled standards like Brand Indicators for Message Identification (BIMI), organizations can display their brand logos next to the subject line of their emails within the recipient’s email inbox — boosting brand presence while providing assurance that the email can be trusted.

To learn more about securing your email and protecting your brand from getting impersonated in email attacks, read Getting Started with DMARC, an ebook from Agari.




Agari is the Trusted Email Identity Company™, protecting companies and people from phishing and socially-engineered email attacks.