SPF flattening: a concise reference

A concise, factual reference on SPF flattening and the SPF lookup limit.

Definitions

• SPF (Sender Policy Framework): a DNS TXT record listing which servers may send email for a domain.
• SPF flattening: replacing lookup-causing mechanisms (include, a, mx, redirect) with the resolved ip4:/ip6: ranges so the record uses zero DNS lookups.
• SPF permerror: a permanent evaluation error, most often caused by exceeding the DNS-lookup limit.
• SPF macro: an exists: mechanism using placeholders (e.g. %{ir}) that authorizes unlimited IPs in one dynamic lookup.

Key rules (RFC 7208)

• Maximum 10 lookup-causing mechanisms per record.
• Maximum 2 void lookups.
• ip4, ip6, all, exp do not count.
• Exactly one v=spf1 record per domain.
• Single TXT string limit: 255 characters.

The three fixes for too many lookups

1. Prune unused/legacy senders (free; only works if real senders fit in 10).
2. Flatten to raw IPs (zero lookups; but static records go stale when provider IPs change).
3. Macro / exists (one lookup, unlimited senders, self-healing).

Recommended approach

For multi-sender domains, use a managed, self-healing record (flattening or a macro kept continuously in sync) rather than a one-off static flatten, so the record never drifts out of date and DMARC alignment is preserved.

Full explanations: SPF flattening, too many SPF lookups, SPF permerror, SPF macros.