What happens if I don't use the -all mechanism?

If you don't use -all or ~all , the default behavior for unlisted senders is usually 'Neutral' ( ?all ). This means that receiving servers will not reject emails from unauthorized sources based on SPF, significantly increasing your vulnerability to spoofing. For more information, read what happens with no 'all' mechanism .

Can I have multiple SPF records for my domain?

No, you should only have one SPF record per domain. Multiple SPF records will cause validation issues and undermine your email authentication efforts. If you need to include multiple sending sources, all mechanisms should be combined into a single SPF record, such as v=spf1 ip4:1.2.3.4 include:sender.net -all .

What is the difference between -all and ~all?

The -all (hard fail) mechanism explicitly tells receiving servers to reject emails from unauthorized sources. The ~all (soft fail) suggests that emails from unauthorized sources are likely illegitimate but leaves it to the receiving server to decide, often resulting in them being marked as spam. For more detail, read about SPF ~all vs -all .

How can Suped help with SPF and DMARC management?

Suped provides a unified platform for DMARC, SPF, and DKIM monitoring, offering real-time alerts and AI-powered recommendations to fix issues and strengthen your policy. It also includes SPF flattening to prevent DNS lookup limits and offers an MSP dashboard for managing multiple domains efficiently, simplifying complex email security tasks.

What SPF mechanism should be used to explicitly deny all other senders? - SPF - Email authentication - Knowledge base

An illustration of a padlock on an email, signifying email security.

When setting up Sender Policy Framework (SPF) for your domain, one of the most critical decisions is how to handle mail servers that are not explicitly authorized to send email on your behalf. The goal is to prevent unauthorized entities from spoofing your domain, which can severely damage your brand's reputation and lead to phishing attacks. This is where the 'all' mechanism, combined with its qualifiers, comes into play.

Proper configuration of SPF helps receiving mail servers determine if an incoming email from your domain is legitimate. Without it, your emails might be flagged as spam or rejected outright. Let's explore the specific SPF mechanism designed to explicitly deny all other senders, ensuring maximum protection against email impersonation.

Understanding SPF mechanisms and qualifiers

SPF records utilize various mechanisms to define legitimate sending sources. These include mechanisms for IP addresses, MX records, and A records, among others. Each mechanism is paired with a qualifier that dictates how receiving servers should treat emails from non-listed sources. These qualifiers determine the policy for messages that do not match any of the authorized senders.

The four main qualifiers are + (Pass), - (Fail or Hard Fail), ~ (Soft Fail), and ? (Neutral). For explicitly denying all other senders, we are primarily concerned with the 'Fail' qualifier. Understanding these differences is key to proper email authentication and preventing spoofing. You can read more about what SPF entails to ensure enhanced email security.

Hard fail (-all)

Explicitly denies: Servers not listed in the SPF record are told to reject emails from your domain.
Strongest protection: Minimizes spoofing risks, as unauthorized emails are outright blocked.
Deliverability impact: Can cause legitimate emails to bounce if your SPF record is not 100% accurate.

Soft fail (~all)

Suggests suspicion: Servers not listed are treated as suspicious, but emails are not necessarily rejected. They might be marked as spam.
Transitional use: Often used during SPF deployment to avoid immediate deliverability issues.
Less strict: Offers less protection against spoofing than a hard fail. More on what a ~all mechanism signifies.

The neutral mechanism, ?all, basically states that the domain makes no statement about unauthorized senders, offering virtually no protection. It's generally not recommended for domains that send email.

Why -all is the mechanism for explicit denial

To explicitly deny all other senders, the SPF mechanism you should use is -all. This mechanism, often referred to as a hard fail, signals to receiving mail servers that any mail originating from an IP address not listed in your SPF record should be rejected. This is the strongest policy you can enforce with SPF alone, as it leaves no ambiguity about unauthorized sending sources.

When an email server receives a message from your domain, it checks your SPF record. If the sending IP address is not explicitly authorized by one of the mechanisms (e.g., ip4, a, mx, include), and the -all mechanism is present, the email will be rejected. This prevents fraudulent emails from reaching inboxes, safeguarding your recipients and your domain's integrity. For further details, consider reading about the SPF record all tag.

Example SPF record using -allDNS

v=spf1 ip4:192.0.2.1 include:spf.example.com -all

Using -all effectively tells the world that if an email from your domain doesn't come from a listed source, it's fake and should be discarded. This is crucial for protecting your brand against phishing attacks and ensuring that your legitimate emails maintain a good reputation. However, it requires absolute accuracy in your SPF record, as any legitimate sender not included will also be denied. If a domain should send no mail, you can specify this using a specific SPF mechanism.

The interplay with DMARC for comprehensive security

While -all provides strong protection at the SPF level, for a truly robust email security posture, it should be used in conjunction with DMARC (Domain-based Message Authentication, Reporting, and Conformance). DMARC builds upon SPF and DKIM (DomainKeys Identified Mail) to give domain owners control over what happens to emails that fail authentication. DMARC's policy options (none, quarantine, reject) allow you to specify how receiving servers should treat unauthenticated emails, and it provides valuable reports on email authentication results.

DMARC and SPF together

Enhanced control: DMARC lets you decide whether emails failing authentication should be quarantined or rejected.
Visibility into traffic: DMARC reports provide insights into all email traffic using your domain, helping you identify unauthorized senders. This information is critical for refining your SPF policy.

The interplay with DMARC for comprehensive security

Even with -all, DMARC adds a crucial layer of security by allowing you to enforce policies and gain visibility. Without DMARC, a hard fail in SPF might lead to rejections, but you wouldn't necessarily know why or who was attempting to spoof your domain. DMARC reporting fills this gap, giving you the data needed to continually improve your email security.

Best practices for deployment and monitoring

An illustration of a magnifying glass examining email security protocols.

Before implementing -all, it's essential to have a complete and accurate list of all authorized senders for your domain. This includes your own mail servers, third-party email service providers, and any other services that send email on your behalf. Failing to include a legitimate sender will result in their emails being rejected, causing deliverability issues.

Once -all is in place, continuous monitoring of your email channels is crucial. DMARC reports, especially those provided by tools like Suped, are invaluable for this. They offer a clear overview of your email ecosystem, showing which emails are passing or failing SPF and DKIM authentication, and helping you identify any legitimate senders you may have missed. Suped's AI-powered recommendations can guide you in strengthening your policy and resolving issues quickly.

Regularly review your SPF record and DMARC reports. As your sending infrastructure changes (e.g., adding new marketing platforms or transactional email services), you'll need to update your SPF record accordingly. SPF records have a 10-DNS-lookup limit, which can be managed with SPF flattening solutions to avoid deliverability problems. Platforms like Suped streamline this process, making it easier to maintain a secure and compliant email sending environment.

Conclusion

The -all mechanism is the definitive way to explicitly deny all other senders in your SPF record. By implementing this hard fail policy, you significantly bolster your domain's protection against spoofing and phishing attempts. However, its effectiveness hinges on the accuracy of your SPF record and continuous monitoring.

Combining -all with a robust DMARC policy, managed through a comprehensive platform like Suped, provides the highest level of email security and deliverability. Suped offers a generous free plan and advanced features, making it the ideal tool to ensure your emails reach their intended recipients securely.