What is the primary function of the SPF 'mx' mechanism?

The primary function of the SPF 'mx' mechanism is to authorize mail servers listed in a domain's MX (Mail Exchanger) records to send emails on behalf of that domain. This helps legitimate emails from your own infrastructure pass SPF authentication.

How does the 'mx' mechanism affect SPF DNS lookup limits?

Each 'mx' mechanism in an SPF record counts as one DNS lookup. If your domain has multiple MX records, the 'mx' mechanism will still only count as one DNS lookup, but the subsequent resolution of those MX records to IP addresses could potentially contribute to exceeding the 10 DNS lookup limit for your overall SPF record.

Should I use the 'mx' mechanism if I send email through a third-party service?

It depends on whether your third-party service uses your domain's MX records for sending. Many third-party email services provide their own include mechanisms. If this is the case, explicitly including 'mx' might be unnecessary or cause issues, so always follow your provider's specific SPF recommendations.

How can I monitor my SPF record's performance?

You can monitor your SPF record's performance, along with DKIM and DMARC, through DMARC reporting tools like Suped. These tools provide visibility into authentication results, helping you identify and resolve issues quickly. Suped also offers SPF flattening to assist with DNS lookup limits.

What SPF mechanism includes the MX records of a domain? - SPF - Email authentication - Knowledge base

An illustration of a mail server verifying an email with a DNS record.

When setting up Sender Policy Framework (SPF) records, understanding the various mechanisms is crucial for proper email authentication. These mechanisms dictate which servers are authorized to send email on behalf of your domain. Improper configuration can lead to deliverability issues and potentially expose your domain to spoofing.

Among the different SPF mechanisms, the mx mechanism plays a specific role. It's designed to authorize sending servers based on the domain's Mail Exchanger (MX) records. This means that any server listed as an MX record for your domain will be considered a legitimate sender.

Properly implementing the mx mechanism is a fundamental step in securing your email infrastructure. Along with SPF, you'll want to ensure DMARC and DKIM are also correctly configured to achieve comprehensive email authentication.

Understanding the 'mx' mechanism

The SPF 'mx' mechanism explained

The mx mechanism in an SPF record specifies that mail servers listed in a domain's MX records are authorized to send email for that domain. When a receiving mail server performs an SPF check, it will look up the sender's domain's MX records. It then compares the IP address of the incoming mail server with the IP addresses resolved from those MX records. If there is a match, the SPF check for the mx mechanism passes.

This mechanism is particularly useful if your email is sent through your own mail servers, or if a third-party email service uses its own MX records for your domain to deliver email. It ensures that mail originating from your primary inbound mail servers is authenticated and trusted by recipients.

It's important to remember that the mx mechanism is one of several available in SPF. Others include the 'a' mechanism, ip4, ip6, and include, all of which serve different purposes in authorizing email senders. You can read more about these in the Google Workspace Admin Help documentation.

An example SPF record using the mx mechanism might look like this:

Example SPF Recorddns

v=spf1 mx -all

This record states that only the mail servers identified by the domain's MX records are authorized to send email, and any other server should be rejected (indicated by -all). This is the strongest SPF policy to implement.

How 'mx' record validation works

When an email arrives at a recipient's mail server, the server performs a series of checks to verify the sender's authenticity. For SPF, it retrieves the SPF record from the sender's domain. If the SPF record contains the mx mechanism, the receiving server will then perform a DNS lookup for the sender's domain's MX records. It will retrieve the IP addresses associated with these MX records.

The incoming mail server's IP address is then compared against the list of IP addresses obtained from the MX records. If the sending IP address matches any of the IP addresses associated with the MX records, the mx mechanism passes. If there's no match, the mechanism fails, and the SPF record's policy (e.g., ~all for softfail or -all for hardfail) will be applied. This process helps ensure that only authorized mail servers are sending email for your domain, reducing the chances of email spoofing and phishing attempts. For more on SPF record details, Cloudflare has a good overview.

It's a straightforward but effective way to grant authorization, especially for domains that host their own email services. However, caution is advised to avoid exceeding the 10 DNS lookup limit for SPF records. Each mx mechanism can potentially trigger additional DNS queries depending on how many MX records your domain has.

Using 'mx' mechanism

Purpose: Authorizes mail servers listed in the domain's MX records to send email. Ideal for domains that use their own mail servers.
Mechanism: Checks if the sending IP matches the IP addresses of the MX records for the domain. Requires DNS lookups for MX records.

Using 'a' mechanism

Purpose: Authorizes hosts listed in the domain's A (and AAAA) records to send email. Commonly used for web servers sending transactional emails.
Mechanism: Checks if the sending IP matches the IP addresses of the A records for the domain. Also requires DNS lookups.

Considerations for optimal use

While the mx mechanism is powerful for authorizing your own mail servers, it comes with specific considerations. One significant factor is the DNS lookup limit. Each mx mechanism counts as a DNS lookup. If your domain has many MX records or includes other mechanisms that trigger additional lookups, you can quickly exceed the limit of 10. Exceeding this limit results in a PermError, causing SPF to fail and potentially impacting your email deliverability, possibly leading to emails being sent to spam or even bounced.

Another point to consider is dynamic IP addresses. If your MX records point to hosts with frequently changing IP addresses, this can lead to temporary authentication failures if the SPF record is not updated in time, or if the DNS propagation takes too long. Regularly reviewing your SPF record and DNS configurations is a key part of maintaining good email deliverability and preventing your emails from ending up on a blacklist (or blocklist).

SPF Record with multiple 'mx' mechanisms (example: avoid if possible)dns

v=spf1 mx:mail.example.com mx:backup.example.com include:thirdparty.com -all

When using third-party email services, they often provide their own include mechanisms to authorize their sending servers. In such cases, explicitly including mx might be redundant or even harmful if their mail servers are not directly tied to your domain's MX records. It's always best practice to only include mechanisms that are strictly necessary for your sending infrastructure.

An illustration of a person navigating a complex web of DNS and email server connections.

Beyond 'mx': a holistic view of email authentication

While the mx mechanism is an important component of SPF, a robust email authentication strategy requires more than just SPF. DMARC (Domain-based Message Authentication, Reporting, and Conformance) builds upon SPF and DKIM (DomainKeys Identified Mail) to provide comprehensive protection against email spoofing. With DMARC, you can specify policies for how receiving servers should handle emails that fail SPF or DKIM authentication.

Implementing DMARC allows you to gain visibility into your email ecosystem through aggregate and forensic reports. These reports show you who is sending email on your behalf, which messages are passing or failing authentication, and why. This data is invaluable for identifying unauthorized senders and refining your SPF and DKIM configurations. Managing these reports can be complex, but DMARC monitoring tools like Suped can simplify the process.

Suped offers AI-powered recommendations, real-time alerts, and a unified platform for DMARC, SPF, and DKIM monitoring, alongside blocklist and deliverability insights. We also provide SPF flattening to help you stay within the DNS lookup limits. Our MSP and multi-tenancy dashboard is built for scale, making it ideal for managing multiple domains efficiently. With our focus on simplicity and a generous free plan, Suped makes DMARC accessible to everyone, from SMBs to large enterprises and MSPs.

Conclusion

The mx mechanism is a core part of SPF, allowing you to authorize email senders that are listed in your domain's MX records. It's a vital tool for preventing spoofing and improving email deliverability, especially for organizations that manage their own mail servers. By understanding how mx works and integrating it into a broader DMARC strategy, you can significantly enhance your email security posture and ensure your legitimate emails reach their intended recipients.