RFC 7208 limits SPF to 10 DNS lookups. What counts:
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include: → 1 lookup (then recursively processes included record)
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redirect= → 1 lookup (replaces current SPF with redirected record)
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mx or mx:domain → 1 lookup (regardless of IPs
resolved)
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a or a:domain → 1 lookup
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ptr or ptr:domain → 1 lookup (deprecated)
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exists:domain → 1 lookup
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ip4: and ip6: → 0 lookups (literal IPs)