SPF Softfail Vs SPF Hardfail: Difference And Best Practices

Learn the difference between SPF Softfail and Hardfail, best practices, and how to secure your business email with PowerDMARC. Complete guide for IT managers and email administrators.

Key Takeaways

SPF hardfail indicates that an email sender is not authorized, often resulting in rejection of the email.
SPF softfail suggests that the email may still be from an unauthorized sender but does not outright reject it, allowing for further review.
Implementing strict SPF policies is essential to avoid unauthorized email impersonation and protect brand reputation.
Combining SPF with DMARC adds an essential layer of security, enabling better handling of email authentication failures.
Regularly monitoring SPF authentication results can help maintain an optimal email security posture and prevent potential vulnerabilities.

With SPF (Sender Policy Framework), you have the flexibility to configure your system to respond to authentication failures in one of two ways: Hardfail or Softfail. In this blog, our experts at PowerDMARC will walk you through the differences between SPF hardfail and softfail, how to configure each, and real-world use cases for IT and security teams. So let’s dive right in!

What is an SPF Failure?

Before diving into the differences between hard fail and soft fail, it’s essential to understand what constitutes an SPF failure and how SPF authentication works.

SPF (Sender Policy Framework) is an email authentication protocol that allows domain owners to specify which IP addresses are authorized to send emails on behalf of their domain. When an email is received, the receiving server checks the sender’s IP address against the domain’s SPF record to determine if the sender is authorized.

The SPF check is performed against the return path domain, which represents the given identity being authenticated. It is crucial to have the SPF record published correctly to ensure proper SPF authentication and avoid email deliverability issues.

SPF Authentication Results

Pass: The sender’s IP is authorized in the SPF record
Fail (Hard Fail): The sender’s IP is explicitly not authorized (-all)
Soft Fail: The sender’s IP is probably not authorized (~all)
Neutral: No policy statement about the sender (?all)
None: No SPF record exists for the domain
TempError: Temporary DNS lookup failure
PermError: Permanent error in SPF record syntax

Each SPF result is an explicit statement about the authorization status of the sender. A ‘fail’ is a strong explicit statement that the sender is not authorized, while a ‘softfail’ is a weak statement indicating probable but not definitive lack of authorization.

An SPF failure occurs when the authentication check returns a “Fail” or “Soft Fail” result, indicating that the sending server may not be authorized to send emails for that domain.

SPF Hard Fail vs Soft Fail: Key Differences Explained

The table provided below explains the basic difference between SPF hardfail and softfail.

SPF Syntax	Type of Fail	Status	Corresponding action taken by the recipient	Typical Use Case	Impact on Deliverability	Recommended For
v=spf1 include:domain1.com -all	Hardfail	Sender unauthorized	Email may be rejected	Production domains with strict security	High risk of blocking legitimate emails	Mature email infrastructures
v=spf1 include:domain1.com ~all	Softfail	Sender may be unauthorized	Email is delivered but marked as suspicious or potentially fraudulent	Testing phase or complex email setups	Lower risk, emails still delivered	Organizations with third-party senders

Note: The hardfail mechanism (‘-all’) is designed to provide maximum protection against phishing and forged emails by causing message rejection for unauthorized senders. However, an incorrect configuration can result in undeliverable email and a 100% bounce rate for unauthorized sources.

In SPF record syntax, only the IP address or addresses specified in the SPF record are authorized to send email on behalf of the domain. All other sources are considered unauthorized senders and may be blocked or marked as suspicious, depending on the fail type.

SPF Hardfail Vs Softfail: As Defined in RFC

According to RFC 7208:

A “Fail” result for SPF directly indicates that the sender of the email is not authorized by the sending domain. “Fail” is synonymous with the Hardfail result (-all) which is clearly indicative of unauthorized domain usage.
However, a much more relaxed approach is configuring SPF Softfail, which indicates a “probable” indication of unauthorized domain usage.

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Recipient Failure Handling for Hardfail Vs Softfail

In section 8.4, RFC defines the following result handling scenarios for SPF hardfail vs for SPF softfail results:

1. SPF Hardfail / Fail

For the “Fail” or hardfail result, your recipient server may choose to reject the email that is unauthorized. If it’s an SMTP transaction, a 550 5.7.1 error code should be returned with an appropriate error description.

IF the recipient server does not reject the email during the SMTP transaction then, RFC recommends receivers to record the SPF results in the Received-SPF or Authentication-Results header.

2. SPF Softfail

As a more flexible policy, Softfail indicates that the administrative management domain does suspect the email to be unauthorized, however it doesn’t wish to outright reject it. In this case, the message is delivered, but with a warning for further review.

Which SPF Failure Mode Should You Use?

Most domains, including those of major organizations and technology companies, use a mix of SPF policies depending on their specific needs and email practices. Choosing between SPF hard fail and soft fail depends on your organization’s email infrastructure, security requirements, and risk tolerance. Here’s a decision framework to help you choose:

Choose SPF Hard Fail (-all) When:

You have complete control over all email sending sources
Your email infrastructure is mature and well-documented
Security is the top priority over deliverability
You rarely use third-party email services
You have comprehensive DMARC monitoring in place

Choose SPF Soft Fail (~all) When:

You’re implementing SPF for the first time
You use multiple third-party email services
Email deliverability is critical for business operations
You have complex email forwarding or relaying setups
You’re still identifying all legitimate email sources

Common Use Cases for SPF Hard Fail and Soft Fail

SPF Hard Fail Use Cases:

Financial institutions: Banks and credit unions requiring strict email authentication
Government agencies: Organizations with high security requirements
Brand protection domains: Domains used solely for brand protection
Transactional email domains: Dedicated domains for automated emails

SPF Soft Fail Use Cases:

Marketing domains: Domains using multiple email marketing platforms
SaaS companies: Organizations with complex email infrastructures
Educational institutions: Universities with multiple departments and email systems
Testing environments: Domains in SPF implementation phase

Real-world scenario: If you’re an IT manager overseeing email security for a mid-sized SaaS company using Google Workspace, Salesforce, and Mailchimp, you might start with a soft fail to ensure all legitimate emails are delivered while you monitor and refine your SPF record.

When Should You Use SPF Hard Fail or Soft Fail?

In the case of SMTP email relaying, you may consider SPF softfail as a safer bet against hardfail. Let’s find out how:

SMTP email relaying is the automatic transfer of messages from one server to another delivery. This means that the email is handed over to a server whose IP address is not listed within your domain’s SPF record. This makes it an unauthorized sender for your email, although, in practicality, it is legitimate.

Do you have any control over this activity? The answer is no as the email is relayed automatically on your recipient’s side. Under these circumstances, SPF will fail for the relayed emails.

Here’s where having an SPF hardfail policy may land you in trouble! As we already know, the hardfail mechanisms may lead to the rejection of failed messages. Hence, these relayed emails may fail to get delivered if your domain is configured with a hardfail policy.

The worst part? The action taken by the SPF failure handling policy will override DMARC and DKIM authentication results. Essentially, even if DKIM and subsequently DMARC passes – the email may still fail to get delivered.

As specified under RFC 7489 Section-10.1 if SPF checks are performed before DMARC operations, the presence of a “-” prefix on a sender’s SPF mechanism, like “-all”, could lead to immediate rejection of emails. This rejection occurs early in the email handling process, even before any DMARC processing takes place.

Therefore, if an email sender’s SPF policy includes a “-all” mechanism, indicating a strict policy to reject emails that fail SPF checks, it could result in message rejection before any DMARC policies or processing occur. This early rejection can happen regardless of whether the email might eventually pass DMARC authentication.

Hence, under these circumstances, SPF Soft fail triumphs over the Hard fail mechanism. It is a considerably low-risk approach that leaves room for review by simply flagging authorized emails instead of rejecting them.

SPF Bypass and Relaying Vulnerabilities

Understanding potential vulnerabilities in SPF implementation is crucial for maintaining robust email security:

Common SPF Bypass Scenarios:

Email forwarding: Legitimate emails forwarded through third-party services may fail SPF
Mailing list servers: Messages sent through mailing lists often change the sender IP
Cloud email services: Dynamic IP ranges in cloud services can cause SPF failures
Mobile email clients: Emails sent from mobile devices through different networks

Mitigation Best Practices:

Implement DKIM alongside SPF for additional authentication
Use DMARC to handle SPF failures gracefully
Regularly audit and update SPF records
Monitor email authentication reports for anomalies

How Do SPF Records Work?

To implement SPF for your emails, you need to create and publish an SPF record on your domain’s DNS. A typical example of an SPF record is as follows:

v=spf1 include:_spf.google.com ~all

In this SPF record, you are authorizing all emails originating from IP addresses listed in Google’s SPF record. The failure mechanism is defined at the very end of the record (~all), ie. Softfail.

Hence, an SPF record defines the protocol version used, the sending sources you’re authorizing, and the failure mechanism. When you publish this record on your DNS, you ensure that only authorized senders are allowed to send emails on your domain’s behalf. If an unauthorized source tries to impersonate you, SPF fails with the type of failure mechanism defined in your record.

Secure SPF Implementation Strategies

Optimal SPF implementation is essential for safeguarding email communication against unauthorized spoofing and phishing attacks. By following best practices, organizations can enhance their email security posture and protect their brand reputation. Here are some strategies and guidelines for implementing SPF securely:

1. Use an SPF Record Generation Tool

The SPF implementation process begins with record generation. You can create your record manually by having a proper understanding of SPF tags. This method is however prone to human errors. Ideally, you can use our automated SPF generator tool. This helps you create an error-free, accurate SPF record that is customized for your organization’s needs.

2. Use Appropriate SPF Mechanisms

Utilize SPF mechanisms such as “include,” “a,” and “IP4”, to specify the allowed sending sources. Be judicious in selecting mechanisms based on your email infrastructure and ensure they accurately reflect your email sending practices.

3. Maintain and Optimize Your SPF Record

Your Sender Policy Framework record needs to be maintained and optimized to prevent it from malfunctioning. SPF tends to break when your authorized senders exceed the 10 DNS lookup limit on the receiver’s side. To maintain an optimal lookup limit, our hosted SPF solution is your best bet! We help domain owners optimize SPF with a single click, to stay under lookup and void limits and maintain error-free SPF.

4. Combine SPF with DMARC

Deploying DMARC (Domain-based Message Authentication, Reporting, and Conformance) alongside SPF provides an additional (yet essential) layer of security. DMARC enables domain owners to specify email handling policies, including what action to take on emails that fail SPF.

DMARC has shown proven results in minimizing email fraud, compromise and impersonation attacks.

5. Implement Strict SPF Failure-Handling Policies

Configure your record to treat SPF authentication failures with strict policies, such as rejecting or marking emails from domains with failures. To do so, you can either implement SPF hardfail or SPF softfail instead of a neutral policy.

6. Monitor SPF Authentication Results

Implement DMARC reports to monitor SPF authentication results, such as SPF pass, and fail, as well as alignment errors. A DMARC analyzer tool can help you parse and analyze SPF authentication data in an organized, human-readable manner.

Final Words

There is no direct answer to the question “Which is better? SPF hardfail or softfail”. While the hardfail tag can provide you with better security, choosing the correct solution for monitoring your sending sources becomes crucial.

PowerDMARC’s advanced domain authentication and reporting platform provides comprehensive SPF and DMARC solutions for businesses of all sizes.

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Frequently Asked Questions

1. What is the reason for SPF soft fail?

SPF soft fail occurs when an email is sent from an IP address that is not explicitly authorized in the domain’s SPF record, but the domain owner has configured a “~all” mechanism instead of “-all”. This indicates the sender is “probably not authorized” but allows the email to be delivered with a warning flag rather than being rejected outright.

2. What is 550 5.7.0 SPF hard fail?

The “550 5.7.0 SPF hard fail” error occurs when an email server rejects a message because the sender’s IP address is not authorized in the domain’s SPF record that ends with “-all”. This is a permanent failure that prevents email delivery and indicates the receiving server has strict SPF enforcement policies.

3. When would you get an SPF hard fail?

You get an SPF hard fail when: 1) The sending IP is not listed in the domain’s SPF record, 2) The SPF record ends with “-all” (hard fail policy), 3) The email is sent from an unauthorized server or service, 4) There are configuration errors in the SPF record, or 5) The domain has implemented strict email authentication policies.

4. What is the hard fail symbol in SPF?

The hard fail symbol in SPF is the minus sign “-” followed by “all” (written as “-all”). This mechanism tells receiving email servers to reject any emails that don’t match the authorized IP addresses or domains listed in the SPF record. It’s the strictest SPF policy and provides the highest level of email authentication security.

5. Should I use SPF hard fail or soft fail for my business?

For most businesses, start with SPF soft fail (~all) during implementation and testing phases to avoid blocking legitimate emails. Once you’ve identified all authorized sending sources and tested thoroughly, consider moving to hard fail (-all) for maximum security. Organizations with complex email infrastructures or third-party services should carefully evaluate the risk of legitimate email blocking before implementing a hard fail.

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Yunes Tarada

Domain & Email Security Expert at PowerDMARC

Yunes is an Operations Team Lead at PowerDMARC with expert knowledge in email authentication and security. Yunes is a Microsoft-certified Azure Administrator Associate with certifications in CompTIA A+ and many more.

SPF Softfail Vs SPF Hardfail: What’s the Difference?