Sender Policy Framework

What is SPF (Sender Policy Framework)?

Sender Policy Framework (SPF) is an email authentication method that helps to identify the mail servers that are allowed to send email for a given domain. By using SPF, ISPs can identify email from spoofers, scammers and phishers as they try to send malicious email from a domain that belongs to a company or brand.

How does a Sender Policy Framework (SPF) work?

Generally, a Sender Policy Framework:

1. Establishes a policy called an SPF record that outlines which mail servers are authorized to send email from that domain.

2. When an inbound server receives incoming mail, it references the rules for the bounce domain in the DNS and compares the IP address of the incoming mail to the authorized addresses defined in the SPF record.

3. The receiving server then uses SPF record rules to determine if the incoming message will be accepted, rejected, or flagged. 

Importance of a Sender Policy Framework (SPF)

Sender Policy Framework (SPF) plays an important role in email security by helping receiving systems verify whether a message was sent from an authorized source. 

• Reduces domain spoofing: SPF helps prevent unauthorized senders from using your domain in fraudulent emails, making it harder for attackers to impersonate your business.
• Strengthens phishing defense  : By validating approved sending sources, SPF can help limit phishing attempts that rely on forged sender domains.
• Supports better email deliverability: Messages sent from verified sources are less likely to be flagged as suspicious, which can improve inbox placement.
• Protects domain reputation: A properly configured SPF record helps build trust with mailbox providers and reduces the chance of your domain being associated with suspicious email activity.
• Reinforces compliance efforts: For organizations in regulated industries, SPF can support broader email security and governance practices by helping establish sender legitimacy.

Used correctly, SPF helps create a stronger foundation for email authentication and reduces the risk of malicious messages reaching recipients under your domain name.

SPF Record Syntax

An SPF record follows a specific format that tells the receiving mail server which sources are allowed to send email for your domain. It usually starts with the SPF version tag, then lists authorized sending mechanisms, and ends with a policy that tells receivers how to handle mail from unauthorized sources.

A basic SPF record might look like this:

v=spf1 ip4:192.0.2.10 include:spf.examplemail.com -all

In this example:

• v=spf1 identifies the record as an SPF record
• ip4:192.0.2.10 authorizes a specific IPv4 address
• include:spf.examplemail.com confirms a third-party authorized sender
• -all tells any receiving server to reject mail sent from sources not listed in the record

Using the correct syntax is important, because even small formatting errors can cause an SPF check to fail and affect email deliverability.

The limits of Sender Policy Framework

While the Sender Policy Framework offers a certain amount of protection against spam and spoofing, it is not a complete email security solution. A forwarded email will evade an SPF test, and the SPF protocol can't spot email that spoofs only the "from" address – the email address that's visible to users. Additionally, for Sender Policy Framework to work, organizations must keep their SPF records constantly updated – a time-consuming and cumbersome task that gets harder as companies change ISP providers.

Another limitation of SPF email authentication is that it can only check the authenticity of the envelope from the address but cannot identify emails where the sender is spoofing the display name or the header from the address in the message. SPF breaks when a message is forwarded and maintaining and updating SPF records can be a challenge as brands add new mail streams or change ISPs.

How SPF differs from DKIM and DMARC

DMARC, or Domain-based Message Authentication, Reporting & Conformance, offers an improvement on the Sender Policy Framework protocol as well as the DKIM protocol. DMARC prevents spoofing more successfully by requiring that the information in the "from" address align with other information about the sender, and it requires that a message is authenticated with either SPF, DKIM or both. DMARC also improves reporting and provides detail on how messages that fail authentication should be handled.

In the past, implementing DMARC authentication has been a challenge. The protocol can be difficult to deploy and hard to manage, requiring a significant investment of time and resources. To solve this challenge, Mimecast offers Mimecast DMARC Analyzer, an easy-to-use solution for streamlining DMARC implementation and management.

Mimecast DMARC Analyzer

As a 100% SaaS solution, Mimecast DMARC Analyzer helps reduce the time and complexity of enforcing a DMARC policy. DMARC Analyzer acts as an expert guide, helping organizations move toward DMARC authentication quickly and cost-efficiently.

DMARC Analyzer provides self-service tools that enable email administrators to:

• Gain the insight and visibility required before a DMARC reject policy is enforced to make sure email does not get blocked.
• Reduce the time, effort and cost of stopping domain spoofing attacks.
• Rely on user-friendly analyzing software to move toward a reject policy as fast as possible.
• Simplify deployment with a step-by-step approach.
• Achieve enforcement and monitor ongoing performance with easy to use alerts, reports and charts.

Additional email security solutions

In addition to DMARC Analyzer, Mimecast email security solutions include:

• Mimecast Secure Email Gateway. Using multiple detection engines and threat intelligence feeds, Mimecast blocks sophisticated and targeted threats at the gateway, including spear-phishing attacks, zero-day attacks, malware and spam.
• Mimecast Internal Email Protect. To stop threats that have landed internally or that are generated from within email systems, Mimecast scans all internally generated email for malicious links and attachments and suspicious content, detecting the lateral movement of attacks via email from one user to another.
• Mimecast Attachment Protect. Using multiple inspection analytics, Mimecast blocks threats embedded in attachments, using safe file conversion to ensure that users get immediate access to the attachments they need. This service also uses static file analysis, behavioral sandboxing and multiple antivirus engines to neutralize threats.
• Mimecast URL Protect. Mimecast delivers protection for malicious URLs on and off the enterprise network with every click, rewriting URLs in inbound emails and performing real-time scans on every link.
• Mimecast Impersonation Protect. Mimecast scans all inbound email for messages that may be attempting to impersonate a CEO, CFO or other executives as well as trusted partners and well-known brands.

Mimecast also offers a Web Security service that adds monitoring and security at the DNS layer to prevent malicious web activity and DNS spoofing and to block access to business-inappropriate websites.

An easier way to authenticate email

SPF is a foundational part of email authentication because it helps receiving systems verify whether messages are being sent from approved sources. While it is not a complete solution on its own, a properly configured SPF record can reduce spoofing risk, support better deliverability, and strengthen trust in your domain’s email traffic.

To get the best results, SPF should be implemented carefully and used alongside broader protections such as DKIM and DMARC.

Mimecast helps organizations strengthen email security with tools and services that simplify authentication, improve visibility, and reduce the risk of domain abuse.