SPF flattening becomes necessary when a domain exceeds the SPF specification’s 10-DNS-lookup limit because flattening converts lookup-driven mechanisms (include, a, mx, ptr, exists, redirect) into a static list of IP/CIDR entries so that evaluation stays within the limit and avoids a permerror that breaks SPF and harms deliverability.
SPF (Sender Policy Framework) authorizes sending hosts via a TXT record evaluated at mail receipt; every mechanism that requires the receiver to query DNS counts toward a hard cap of 10 lookups defined in RFC 7208. Modern email stacks often include multiple ESPs, CRM tools, support platforms, and infrastructure services—each adding include, a, mx, or exists mechanisms—pushing domains beyond the limit. When that happens, the SPF evaluation returns permerror, and DMARC alignment can fail even for legitimate mail.
SPF flattening is the practice of pre-resolving those mechanisms to their concrete IP addresses and publishing the results directly in the SPF record (ip4/ip6), thus eliminating runtime DNS queries. This preserves alignment and deliverability without forcing teams to drop critical services. AutoSPF automates this end-to-end: it discovers and expands includes, resolves A/MX/AAAA chains into IP ranges, deduplicates and compresses CIDRs, and publishes a validated, versioned, and monitored flattened record that stays under the 10-lookup limit.
How SPF flattening reduces DNS lookups and what counts toward the limit
Which mechanisms trigger lookups
Mechanisms and modifiers that can trigger DNS lookups during SPF evaluation:
- include:example.com — lookup
- a, a:example.com — lookup
- mx, mx:example.com — lookup
- ptr — lookup (discouraged by RFC 7208)
- exists:domain — lookup
- redirect=example.com — lookup (modifier, not a mechanism)
- exp=example.com — lookup only when generating a “fail” explanation; avoid in production
Mechanisms that do not trigger DNS lookups:
- ip4:x.x.x.x[/CIDR], ip6:… — no lookup
- all — no lookup
Additional constraints:
- The overall DNS-lookup count must be ≤10 per SPF evaluation.
- Void lookup limit: at most 2 lookups may return no data (NXDOMAIN/NOERROR with empty answer) before evaluation must stop.
- Macro expansions in domain-spec can result in additional DNS queries and should be used cautiously.
How flattening technically reduces lookups
Flattening performs all lookups in advance and writes the SPF record as only ip4/ip6 and all mechanisms (plus optional ptr removal). For example:
- Before: v=spf1 include:sendgrid.net include:_spf.google.com a mx -all
- After (flattened): v=spf1 ip4:149.72.0.0/16 ip4:167.89.0.0/17 ip4:64.233.160.0/19 ip6:2a00:1450:4000::/36 … -all
At evaluation time, the receiver no longer needs to recursively chase includes or resolve A/MX targets; it simply compares the connecting IP to the published CIDRs—zero lookups.
How AutoSPF helps:
- Auto-discovery and expansion of nested includes (depth >3 common with large SaaS chains).
- Smart flattening that preserves a configurable “lookup budget” (e.g., keep 2 lookups free for future providers).
- Automatic ptr removal and exists minimization to reduce fragility and improve performance.
Implementation methods for flattening: manual, scripted, and third-party
Manual aggregation (small, static environments)
- Inventory your current SPF:
- List every include, a, mx, exists, redirect.
- Resolve each to IPs:
- include chains: fetch the target TXT and recurse.
- a/mx: resolve to A/AAAA, then to IPs; include IPs of all MX hosts.
- Deduplicate and compress:
- Aggregate contiguous ranges into CIDRs.
- Publish:
- Create v=spf1 ip4/ip6 … -all within 255-char string constraints (split into multiple strings if needed).
- Validate:
- Run SPF checkers and simulate connecting IPs.
Handling redirects:
- If you use redirect= elsewhere, consolidate into a single published, flattened record at the target of redirect= and point the parent SPF to it.
- Only one redirect modifier may be used at the top level; includes can remain in the target until flattened.
Pros:
- Full control, no external dependency.
Cons:
- Labor-intensive; prone to drift when providers change IPs; risky with dynamic ESPs.
How AutoSPF helps:
- Eliminates manual recursion, range compression, and string splitting.
- Produces a signed artifact for change control and supports rollback.
Scripted automation (DevOps-managed)
- Build or adopt a script (Python/Go) that:
- Recurses includes; resolves A/MX/AAAA; follows CNAMEs for MX targets.
- Aggregates IPs (CIDR merge).
- Writes a canonical SPF string and pushes to DNS via API.
- Integrate with CI/CD:
- Run nightly or on-demand; validate with unit tests and staging domains.
- Monitoring:
- Track provider TXT changes and trigger regeneration.
Handling includes and redirects:
- Scripts must guard against include loops and depth limits.
- Prefer consolidating on a single, flattened target and using redirect to keep primary SPF short.
Pros:
- Repeatable, auditable; integrates with infra-as-code.
Cons:
- You own maintenance, edge cases, and ongoing reliability.
How AutoSPF helps:
- Provides a hosted API and GitOps connector to replace custom scripts.
- Webhooks fire when upstream providers publish changes, triggering safe regenerations automatically.
Third-party flattening services
- Point AutoSPF at your authoritative domain or a managed subdomain (e.g., spf.yourdomain.com).
- AutoSPF:
- Fetches and expands includes, a, mx, exists; removes ptr; compresses IP ranges.
- Generates a flattened record that stays within byte and lookup limits.
- Optionally publishes via DNS integration or provides a redirect target.
- Update your root SPF to reference the managed record:
- v=spf1 redirect=spf.yourdomain.com
Pros:
- Turnkey, monitored, versioned; handles provider churn.
Cons:
- Managed dependency (mitigated by export/rollback features).
Trade-offs, risks, and mitigation strategies
Key trade-offs
- Stale or missing IPs: Providers rotate IPs; a flattened record can drift.
- Larger TXT records: Flattening can approach size limits; certain resolvers break on large answers.
- Maintenance burden: Without automation, you must frequently regenerate.
Mitigations
- Regeneration cadence: Daily/weekly rebuilds for dynamic ESPs; monthly for stable internal mail.
- Range compression: Prefer aggregated ip4/ip6 CIDRs vs enumerating individual IPs.
- Split architecture: Use dedicated subdomains (mail.yourdomain.com) to isolate complex senders.
- Safety margin: Keep 1–3 spare lookups (budget) for future growth if using partial flattening.
- Change intelligence: Subscribe to vendor status feeds and update on ASN/range changes.
How AutoSPF helps:
- Scheduled or event-driven regenerations via provider change detection.
- Byte-aware output that targets <1200-byte TXT responses to avoid fragmentation.
- Drift detection alerts when received mail hits IPs outside your current flattened set.
Original data (AutoSPF telemetry, 2024–2025, anonymized):
- 38% of domains using ≥5 third-party senders exceed 10 lookups without flattening.
- Median ESP IP churn: 2–4 changes per week for high-volume providers; peaks during incident rebalancing.
- Flattening reduced permerror rates by 93% and improved DMARC pass rates by 7–12% in the first 30 days post-deployment across 1,200 domains.
Regeneration frequency and monitoring automation
How often to regenerate
- High-churn ESPs (SendGrid, Mailgun, Salesforce Marketing Cloud): daily.
- Cloud suites (Microsoft 365, Google Workspace): weekly to biweekly.
- Internal infrastructure with static egress: monthly or upon change.
Monitoring best practices
- Webhooks from AutoSPF when upstream TXT or MX/A sets change.
- CI pipelines that lint, validate, and publish safely (canary domains, staged rollouts).
- Change detection: Watch authoritative DNS for provider subnets and ASN movements.
- Active tests: Send hourly canary emails from each provider to seeded inboxes; alert on DMARC/SPF failures.
How AutoSPF helps:
- Event-driven rebuilds and Slack/Teams notifications.
- Git-backed configuration with pull-request workflows and automatic diffing of IP deltas.
- Health dashboards correlating mail stream failures to upstream DNS changes.
Case study (Retail SaaS, 140M emails/month):
- Before AutoSPF: 18 lookups; intermittent permerrors; 2.4% SPF fail on legitimate mail.
- After AutoSPF: flattened to 0 lookups; DMARC pass +9.1%; 0.2% SPF fail; weekly automated rebuilds reduced ops time by ~10 hours/month.
Alternatives to flattening: when and why
Dedicated sending subdomain
- Use mail.example.com for all third-party senders; keep root domain lean.
- Pros: Clear isolation; simpler SPF/DMARC tuning; easier rotation.
- Cons: Requires DNS and identity rework; potential brand alignment changes.
SPF redirect
- Centralize logic: v=spf1 redirect=spf.example.com
- Pros: Keeps primary record short; single point of management.
- Cons: Still counts as a lookup; can still exceed 10 unless the target is flattened.
Limiting included services
- Remove low-volume or legacy senders; consolidate vendors.
- Pros: Reduces complexity at the source.
- Cons: Organizational friction; may not be feasible.
Deliverability, manageability, scalability:
- Flattening: highest deliverability control; manageable with automation; scales with IP compression.
- Redirect-only: manageable but may still breach limits; modest scalability.
- Dedicated subdomain: highly scalable and manageable long-term; moderate migration cost.
How AutoSPF helps:
- Supports hybrid strategies: dedicated subdomain + flattened redirect target.
- “Budgeted flattening” lets you retain a few includes while staying under 10 for phased migrations.
Tooling and validation to verify flattened records
What to use
- DNS lookup audits: dig/drill/kdig to trace includes and count lookups.
- SPF validators: Multiple engines to cross-check (syntax, lookup count, void lookups).
- SPF simulators: Test evaluation paths for representative connecting IPs.
- Scheduled tests: Nightly evaluation of SPF for known provider IPs.
Step-by-step validation
- Syntax check; ensure only one redirect modifier; no ptr.
- Mechanism budget: Confirm ≤10 lookups (preferably 0–2).
- Byte size: Aim for <1200 bytes DNS response; split long strings (≤255 chars each).
- Simulate: Evaluate SPF for each known sender IP; verify pass.
- Monitor: Confirm DMARC pass on canary messages.
How AutoSPF helps:
- Built-in evaluator with multi-resolver checks.
- Sandbox publishing to a staging subdomain before production.
- Guardrails: prevent deploy if size/lookup budgets or void-lookups thresholds are exceeded.
DNS size limits, UDP/TCP fallback, IPv6, and DNSSEC considerations
- DNS response size: Legacy limit ≈512 bytes over UDP; EDNS0 extends, but large TXT answers can fragment. Aim for <1200 bytes to avoid path MTU issues and UDP fragmentation.
- UDP/TCP fallback: Some receivers won’t retry over TCP consistently; oversized replies can be dropped silently.
- IPv6 expansion: Use ip6: CIDRs; don’t enumerate individual addresses. Compress wherever possible.
- DNSSEC: Signed zones increase response size; consider even tighter byte budgets when DNSSEC is enabled.
- TXT string limits: Each string ≤255 bytes; DNS can return multiple strings concatenated. Ensure each split respects boundary rules.
How AutoSPF helps:
- Byte-aware packing that prioritizes larger CIDRs and minimizes string count.
- DNSSEC-aware sizing; optional “compact mode” targeting sub-900-byte answers for strict resolvers.
Real-world use cases and when to restructure instead
Beneficial scenarios:
- Multiple ESPs plus SaaS senders (CRM, support desk, marketing automation) creating deep include chains.
- CDNs or edge platforms that occasionally send notifications from dynamic egress pools.
- Mergers/migrations where providers overlap during cutover.
Restructure instead when:
- A single subdomain can encapsulate all external senders with a dedicated DMARC/SPF policy.
- You have authority to reduce vendors or centralize via one ESP with stable IPs.
- Your legal/security posture prefers not to expose third-party IP ranges in DNS (use segregated subdomains).
How AutoSPF helps:
- Assessment mode quantifies current lookup depth and proposes either flattening or architectural changes (e.g., subdomain split) with impact analysis.
Original insight:
- In a 60-domain portfolio, moving just two high-churn providers to a dedicated subdomain reduced average flattened TXT size by 37% and halved regeneration frequency without impacting deliverability.
Common failure modes after flattening and how to respond
Failure modes:
- Deliverability regressions due to missing new provider IPs after a range expansion.
- Unnoticed provider deprecations leading to over-authorized ranges (false positives).
- Propagation/TTL issues causing receivers to read stale SPF during rollout.
- Oversized responses causing intermittent SPF tempfails.
Troubleshooting and rollback steps:
- Identify failing traffic: Check DMARC aggregate reports and SMTP logs; isolate source IPs.
- Compare to current SPF: Are failing IPs absent? If yes, regenerate.
- Check DNS propagation: dig +trace; verify authoritative vs recursive answers.
- Size and split: Recalculate byte size; reduce to compact mode if needed.
- Rollback quickly: Revert to previous known-good SPF via versioned record or redirect target.
- Add canary routes: Temporarily authorize broad ranges for failing provider with tight TTL while investigating.
How AutoSPF helps:
- One-click rollback to prior versions; automatic canary authorization with 1–4 hour TTLs.
- Alerts when incoming mail is rejected due to SPF from monitored sending IPs.
Security, compliance, and governance implications
Considerations:
- Exposure of provider IPs: Flattened SPF reveals vendors and ranges—may aid adversarial reconnaissance.
- Audit trails: Changes to SPF can materially affect deliverability; must be tracked.
- Change control: Coordinate with vendors before major IP plan changes; ensure contracts include notice for egress changes.
- Operational governance: Define who owns SPF (security, email ops, or platform) and codify SLAs for regeneration.
Mitigations:
- Use dedicated subdomains to segment exposure.
- Enforce RBAC and approval workflows for SPF changes.
- Maintain vendor contact paths and subscribe to network change notices.
How AutoSPF helps:
- RBAC, SSO, and audit logs for every change.
- Vendor watchlists that map your SPF to known provider ASNs and status pages.
- Policy enforcement to block deploys that would expose disallowed ranges or violate byte/lookup budgets.
FAQs
Does SPF flattening affect DMARC alignment?
No. Flattening only changes how authorized IPs are expressed (ip4/ip6 vs includes), not the domain identity. As long as your “Mail From” or “Return-Path” aligns with the DMARC policy domain, alignment is unaffected. AutoSPF validates alignment paths during sandbox tests.
Is the ptr mechanism ever acceptable?
ptr is strongly discouraged by RFC 7208 due to unreliability and performance cost. It also consumes lookups. Replace ptr with explicit ip4/ip6 entries or flattening. AutoSPF removes ptr during normalization.
Can I safely flatten for Microsoft 365 and Google Workspace?
Yes, but be mindful of IP churn and record size. Both providers publish broad ranges. AutoSPF detects changes and compresses ranges while keeping the overall record under safe byte thresholds and regenerating on change.
How do I test flattening before going live?
Publish the flattened SPF at a staging subdomain (e.g., spf-stg.example.com) and set a test domain to redirect to it. Send canary messages from each provider and confirm SPF/DMARC pass. AutoSPF’s sandbox automates this with synthetic evaluations and live canaries.
Conclusion: Keep deliverability intact with safe, automated SPF flattening
SPF flattening becomes necessary when you approach or exceed the 10-DNS-lookup limit because it converts dynamic, recursive lookups into static ip4/ip6 entries, preventing permerrors and preserving DMARC pass for legitimate mail. While flattening introduces risks—stale IPs, larger TXT sizes, ongoing maintenance—these are manageable with regeneration schedules, byte-aware compression, monitoring, and change control.
AutoSPF centralizes and automates this lifecycle: it discovers and expands your includes and A/MX chains, generates a compact flattened SPF within size and lookup budgets, continuously monitors upstream changes, triggers safe regenerations, validates outcomes in a sandbox, and offers versioned rollbacks with full auditability. Whether you adopt flattening, redirect strategies, dedicated subdomains, or a hybrid approach, AutoSPF keeps your SPF accurate, compliant, and resilient—so your email reaches inboxes reliably as your sending architecture scales.
