Best Practices

SPF Flattening Explained

SPF flattening replaces nested includes with explicit IP mechanisms to reduce DNS lookups and avoid the ten-lookup limit.

Advanced · 8 min read · Reviewed Jul 4, 2026

Quick answer

SPF flattening expands nested include chains into explicit ip4 and ip6 mechanisms so receivers perform fewer DNS lookups during evaluation. This helps domains stay under the ten-lookup limit. Flattened records require ongoing updates when provider IPs change, so automation or managed services are often used to keep flattened policies accurate without manual DNS edits.

Beginner explanation

Flattening sounds technical, but the idea is simple: instead of asking receivers to follow a chain of includes, you publish the IP addresses directly in your SPF record. That trades a longer record for fewer DNS queries.

Organizations flatten when they approach the ten-lookup ceiling or when they want predictable evaluation without depending on a vendor's nested SPF policy. It is a common technique for busy marketing domains with many ESPs.

Flattening is not free maintenance. IP ranges change. A flattened record that is not updated becomes stale and can fail legitimate mail when providers shift infrastructure.

Technical explanation

Manual flattening involves resolving each include to its terminal ip4 and ip6 mechanisms, deduplicating ranges, and publishing them at your domain. Automated flattening services periodically refresh IP lists and rewrite DNS on your behalf, sometimes combined with a single include to the flattener's hostname.

Flattening reduces lookup depth but increases TXT record size. DNS operators must watch the 512-byte UDP response limit and split long strings appropriately. Some receivers truncate or mishandle oversized responses, so test with multiple public resolvers after publishing.

Flattening does not remove the need for includes entirely when providers rotate IPs faster than you can operationalize updates. Hybrid models keep stable office IPs explicit while delegating volatile ESP ranges to a managed include or automated flattener.

Business impact

Done well, flattening restores reliable SPF and DMARC pass rates for domains that outgrew traditional include-only policies. Campaign and employee mail stabilize without emergency removals of legitimate vendors.

Done poorly, flattening creates silent failures when IPs change. Operations teams may not notice until DMARC reports show sudden SPF fail spikes across multiple sending channels.

Common mistakes

- Flattening once and never refreshing after provider IP expansions
- Duplicating the same IP ranges from multiple includes without deduplication, bloating TXT size
- Flattening while still publishing redundant includes that negate lookup savings

How SPF Manager helps

SPF Manager shows lookup counts before and after simulated flattening so you can measure benefit against maintenance cost. It highlights which includes are safest to flatten based on stability and overlap.

Monitoring detects when flattened IP ranges diverge from live provider policies, prompting refresh before receivers start failing mail.