exists — SPF Mechanism

The SPF exists mechanism passes if a given domain resolves to any A record. Used with macros for dynamic per-IP whitelists.

Type
Mechanism
Syntax
exists:<domain>
DNS Lookup
Yes — counts toward 10-lookup limit
Example
exists:%{ir}.whitelist.example.com

💬 What This Mechanism Does

The exists mechanism passes if the specified domain resolves to at least one A record (any IP). By itself this is rarely useful, but combined with SPF macros (like %{i} for the sending IP or %{d} for the sending domain) it enables powerful dynamic lookups — for example, checking whether a specific IP is on a custom allowlist.

When to Use This

  • Dynamic per-IP whitelists maintained as DNS zones
  • Rate-limiting or geographic restrictions encoded in DNS
  • Rare advanced use cases for large ESP infrastructure

⚠️ Watch Out For

  • Counts as one DNS lookup.
  • Requires SPF macro knowledge to be useful.
  • Not widely understood — document it carefully if you use it.

📚 RFC References

↗RFC 7208 §5.7 — The "exists" Mechanism ↗RFC 7208 §7 — Macros

🔗 Related SPF Elements

include
Mechanism

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