What Is the DNS Lookup Limit (SPF)?

What is the DNS Lookup Limit (SPF)?

The DNS Lookup Limit in SPF refers to the rule that an SPF evaluation may perform at most 10 DNS-querying mechanisms and modifiers during a single check. This cap exists to prevent excessive or recursive DNS lookups that could slow down mail delivery or create abuse risks. If an SPF policy requires more than 10 lookups, SPF processing fails with a permerror result and receivers typically treat the message as failing SPF.

This constraint affects how you compose your v=spf1 record. Certain mechanisms trigger DNS queries while others do not. Careful planning keeps your record within the limit and ensures reliable authentication results across mailbox providers.

How the DNS Lookup Limit Works

During SPF evaluation, the receiving server expands your v=spf1 policy. Each time expansion requires an external DNS query, it counts toward the limit of 10. The count accumulates across includes and redirects, not just the top level.

Mechanisms and modifiers that trigger DNS lookups:

• a
• mx
• ptr
• exists
• include
• redirect

Mechanisms that do not trigger DNS lookups:

• all
• ip4
• ip6
• exp

There is also a separate limit on void lookups. If two or more DNS-querying mechanisms return no data or NXDOMAIN, evaluation may stop with a permerror. This protects receivers from expensive or abusive policies.

Example of a policy at risk of exceeding the limit:

Even if the top level appears to have fewer than 10 lookups, each include can itself contain additional include, a, or mx mechanisms. The total across the entire expanded tree must remain at or below 10.

Why the Limit Is Important for Deliverability and Security

If your SPF policy exceeds the 10 lookup limit, receivers can return a permerror and treat the message as if SPF failed. Depending on DMARC policy and alignment, that failure can reduce inbox placement or cause outright rejection for protected domains.

Operational risks of hitting the limit include:

• Intermittent SPF failures when vendors change their includes
• Hard to diagnose delivery issues across different receivers
• Unexpected DMARC failures due to SPF misalignment
• Increased fragility when adding new sending services

Keeping the policy within limits ensures stable authentication outcomes, reduces support overhead, and preserves domain reputation. It also guards against attackers crafting policies that force expensive resolver work on recipients.

Practical Ways to Stay Under the Limit

Design SPF policies to minimize DNS lookups without losing accuracy. Effective tactics include:

• Prefer ip4 and ip6 where feasible to encode known ranges directly
• Consolidate overlapping vendor includes and remove unused services
• Avoid ptr and use a and mx only when necessary
• Use vendor provided aggregate includes when available rather than chaining multiple brand specific includes
• Flatten cautiously by resolving includes to IPs when ranges are stable, with a process to recheck on vendor changes
• Use redirect once for clean inheritance instead of multiple layered redirects
• Monitor for void lookups and fix typos or decommissioned hostnames

A streamlined example that respects the limit:

DNS Lookup Limit and DMARCeye

DMARCeye evaluates your SPF policy with full expansion to estimate total DNS lookups and highlight policies that approach or exceed the limit. The platform tracks which mechanisms contribute most to the count and surfaces risky chains of include and redirect.

DMARCeye also flags void lookups, deprecated ptr usage, and misconfigurations that could push your record over the threshold after a vendor update. With clear recommendations and change tracking, teams can simplify SPF policies, stay within the 10 lookup rule, and maintain consistent DMARC pass rates.

Sign up for a free trial of DMARCeye today and secure your email domain.

To learn more about DMARC and DMARC-related terms, explore the DMARCeye Glossary.