To test an SPF flattener’s compatibility with DMARC and DKIM, first publish the flattened SPF in a non-authoritative “shadow” label, run DNS and lookup-budget checks, simulate end-to-end mail flows from every sender with DKIM signed, validate SPF/DKIM pass and DMARC alignment under relaxed/strict modes, and monitor aggregate and real-time metrics before and after a phased cutover with a pre-defined rollback.
Context and background
An SPF flattener rewrites your “v=spf1 …” TXT record by expanding include chains into explicit IP ranges, reducing or eliminating DNS lookups at evaluation time. Flattening helps avoid SPF’s 10-lookup cap, but it can introduce new risks: oversized TXT records, missed vendor ranges, stale IPs, or misapplied redirect/modifiers. Because DMARC bases its pass/fail on alignment against SPF or DKIM, any SPF permerror/temperror or alignment mismatch can cascade into DMARC failures unless DKIM remains healthy and aligned.
In a multi-sender environment (e.g., Microsoft 365, a marketing ESP, a transactional API sender, and a support ticketing platform), testing must validate that SPF still passes for every envelope domain and that DKIM signatures survive hops while aligning with the header From. Your goal is to prove that flattening preserves authentication outcomes—and if it changes them, you’ll detect that safely in staging with clear rollback steps.
AutoSPF is built for this exact workflow: it creates a preflight diff of your SPF, manages lookup budgets automatically, simulates expansion safely, and provides a staging/“shadow publish” path with continuous monitoring and one-click rollback to minimize DMARC and deliverability risk.
What an SPF flattener changes—and how that can impact DMARC/DKIM
Testing starts with understanding how flatteners alter mechanics and where DMARC/DKIM can break.
How SPF flatteners alter records
- Replace “include:” chains with explicit ip4/ip6 mechanisms
- Optionally substitute “include:” with a redirect= target or a vendor-owned helper record
- Remove or refactor ptr (deprecated) and exists mechanisms, and sometimes normalize a/mx expansions
- Consolidate overlapping CIDRs; chunk TXT strings to stay within provider limits
AutoSPF performs deterministic expansion, collapses CIDRs, and validates final output against DNS provider limits before you ever publish.
Specific changes that risk DMARC alignment (and how to test)
- Redirect pitfalls: If a flattener switches to “redirect=otherdomain.tld,” SPF alignment still uses the MailFrom/HELO domain—but misconfigurations can lead to receiver bugs or interpretation errors. Test DMARC alignment explicitly under both relaxed and strict alignment (aspf=r/s). AutoSPF supports redirect-free flattening by default and flags any redirect-induced ambiguity.
- Over-expansion or omissions: Missing an ESP’s new IP range = SPF fail = DMARC fail unless DKIM passes. Validate includes with vendor metadata. AutoSPF tracks vendor ranges and alerts on drift.
- Permerror from record size or syntax: Truncation, bad chunking, or malformed macros will permerror SPF. DMARC treats SPF permerror as “fail.” AutoSPF’s linter prevents invalid string breaks and performs parse/permerror simulations.
Macro expansion, modifiers, and canonicalization
- SPF macros (e.g., %{i}, %{h}, %{l}) must generally be left intact if used by vendors; expanding them statically is unsafe. AutoSPF preserves macro semantics and warns when a vendor include relies on runtime macros.
- DKIM canonicalization is independent of SPF flattening, but DKIM must remain robust to ensure DMARC passes even if SPF temporarily fails; see test plan in Section 2. AutoSPF’s DKIM Health checks validate c=relaxed/relaxed vs simple/simple resilience across your senders.
Common failure modes introduced by flatteners
- Exceeded 10-lookup budget due to hidden includes or recursive expansions
- DNS TXT exceeding provider limits (e.g., >255 bytes per string; provider total-size caps)
- TTL mismatches causing stale data during vendor IP rotations
- Include omissions when vendors publish new _spf.* hosts without changelog signals
AutoSPF mitigates these via:
- Preflight budget calculator and recursive dependency graph
- Safe chunking and multi-string assembly validated against 20+ DNS providers’ constraints
- Vendor-watcher that refreshes flattened records before TTL expiry
- Missed-include detection and automatic reflatten jobs
A complete test plan to verify DKIM validity and simulate mail flows
Your testing should prove that DKIM still passes and aligns—and that SPF flattening does not regress DMARC outcomes.
Step-by-step DKIM validation across services
- Inventory senders: Direct MTA, cloud mailbox (e.g., Microsoft 365/Google Workspace), transactional (e.g., Amazon SES, Mailgun), marketing (e.g., SendGrid, Braze), support desk (e.g., Zendesk).
- Confirm DKIM keys/selectors per sender; publish with 1024–2048-bit keys, rotating quarterly. AutoSPF’s Selector Health view checks presence, bit length, and expiry schedules.
- Send test messages from each service:
- Use consistent From: example.com
- Vary DKIM canonicalization (relaxed/relaxed preferred; test simple/simple for sensitivity)
- Include message bodies with minor whitespace/HTML changes to test c=relaxed robustness
- Validate on multiple receivers: Gmail, Microsoft, Yahoo, a corporate gateway with OpenDMARC/OpenDKIM.
- Inspect Authentication-Results:
- dkim=pass header.d=example.com header.s=selector1
- spf=pass smtp.mailfrom=example.com
- dmarc=pass (p=quarantine) header.from=example.com
- Confirm alignment modes:
- relaxed: sub.example.com aligns with example.com
- strict: must be exact-match; change DMARC “aspf=s/adkim=s” temporarily in a shadow policy to test strict mode without production impact (AutoSPF DMARC Lab simulates alignment without enforcing).
AutoSPF provides an in-app Inbox Test that sends to multiple test receivers and aggregates Authentication-Results per sender and selector, flagging any DKIM breakage that could mask SPF regressions.
Simulate end-to-end flows and DMARC outcomes
Define a matrix of scenarios:
- Direct send (Postfix/Exim) with envelope from example.com
- Third-party vendor with subdomain envelope (bounce.mail.vendor.com)
- Marketing ESP using shared pools vs dedicated IPs
- Forwarding and list expanders (ARC presence, body modifications)
For each:
- Validate SPF result and alignment (include strict vs relaxed)
- Validate DKIM signature survival after common transformations (footers, tracking links)
- Confirm DMARC policy outcomes (p=none/quarantine/reject) at different pct= values
AutoSPF’s Mail Flow Emulator replays message samples with canonical changes and records how DMARC would evaluate under your policy, before and after flattening.
Configure DKIM for resilience
- Use c=relaxed/relaxed to tolerate minor header/body rewrites common in mailing platforms
- Maintain at least two selectors per domain for seamless rollovers
- Align d=domain in DKIM to the visible From domain (or exact-match subdomain if using strict)
AutoSPF’s Rotation Assistant sets rotation reminders, checks for orphaned selectors, and alerts if a sender falls back to a non-aligned signing domain.
Deployment best practices: TTL, record length, include mechanics, and lookup budgeting
Flatteners reduce runtime lookups, but you must deploy safely.
TTL strategy
- Staging: Low TTL 300–900s for rapid iteration
- Post-cutover: 3600–7200s to balance cache freshness and propagation stability
- Vendor-sensitive includes: shorter TTL during known IP migration windows
AutoSPF automatically chooses safe TTLs by vendor, shortens TTL before known migrations, and lengthens post-stabilization.
Record size and chunking
- Keep each TXT string under 255 bytes; split across multiple quoted strings
- Ensure total record size fits provider limits (commonly 1–4 KB per TXT record, varies)
- Avoid near-limit records that risk silent truncation
AutoSPF’s Size Guard simulates provider-specific limits and ensures parse-compatibility across major DNS hosts.
Includes, mechanisms, and 10-lookup budget
- Target ≤2–3 lookups after flattening, ideally 0 during receiver evaluation
- Prefer static ip4/ip6 when possible; use “a”/“mx” judiciously (they cascade into lookups)
- Eliminate “ptr” and avoid “exists” unless absolutely necessary
AutoSPF’s Budget Optimizer computes worst-case lookups including recursive includes and caches the flattened output to keep you safely below SPF’s 10-lookup limit.
Strict vs relaxed alignment choices
- Default to relaxed alignment (adkim=r, aspf=r) in production; rely on DKIM as your primary DMARC pass path
- Use strict alignment for high-risk streams; confirm every sender aligns exactly
AutoSPF’s DMARC Planner models alignment outcomes by sender stream and recommends per-stream exceptions or subdomain splits.
Monitoring, logging, and automated validation
Production safety depends on catching regressions quickly.
What to monitor for SPF/DKIM/DMARC
- DMARC aggregate (RUA) trends by source IP and provider
- SPF permerror/temperror rates and top failing HELO/MAIL FROM domains
- DKIM fail/none rates by selector; signature body/hash mismatches
- Alignment failures when using strict modes
- DNS change drift: vendor includes changed since last flatten
AutoSPF ingests RUAs, correlates with your flattener version, and highlights spikes attributable to a specific SPF change.
Alerting thresholds (practical defaults)
- DMARC fail rate >1% over 24h or >3x baseline
- SPF permerror >0.3% sustained for 2h
- DKIM fail rate for any selector >0.5% over 6h
- New source IP observed with SPF fail >100 messages in 1h
AutoSPF ships with opinionated alerts you can tune; all alerts link back to the exact flattener diff.
Build an automated test suite with DNS mocking
Automate unit and integration tests so every flatten run is validated before publish:
- DNS mocking: Use a local resolver (CoreDNS) with zones for example.com and vendor SPF hosts
- SPF evaluation: spf-engine or pyspf in CI to verify pass/fail for representative sending IPs
- DKIM verification: dkimpy to validate signatures on stored fixture messages
- DMARC policy evaluation: OpenDMARC in test mode or a library-based evaluator
- Mail flow emulation: swaks to send test messages through a staging MX
Example pseudocode for CI:
- Unit: Assert flattened TXT compiles, size OK, lookup budget <=2
- Integration: For each sender IP and envelope domain, expect spf=pass; for DKIM fixtures, expect dkim=pass; for DMARC, expect pass under current policy
AutoSPF CI Hooks export the flattened TXT plus a machine-readable plan (JSON) of expected SPF decisions and vendor metadata to drive your tests deterministically.
Risk management: failure modes, differences among flatteners, and safe rollout
Be explicit about what can go wrong, how tools differ, and how you’ll ship safely.
Common failure modes and their DMARC impact
- Exceeded lookups → SPF permerror → DMARC fail if DKIM also fails
- Truncated/malformed TXT → SPF permerror, or “none” → DMARC fail if DKIM fails
- TTL drift/staleness → authorized IPs change and SPF fails → deliverability drop
- Include omissions → a sender suddenly fails SPF → DMARC enforcement quarantines/rejects
AutoSPF’s Preflight simulates these cases and blocks publication when thresholds are breached.
Popular SPF flatteners: differences that affect compatibility
- Open-source scripts (e.g., “spf-tools,” “flatten-spf”) often:
- Expand includes but may not track vendor IP churn or provider size limits
- Ignore macro semantics or complex modifiers
- Provide limited monitoring/rollback
- Commercial services vary:
- Some replace includes with redirect to vendor domains (simpler records but trust/dependency considerations)
- Some maintain near-zero lookups and dynamic updates with SLAs
What AutoSPF does differently:
- Preserves macro semantics, warns on unsafe expansions
- Deterministic chunking for multi-provider DNS compatibility
- Versioned releases with easy rollback and RUAs-linked diff auditing
- Vendor intelligence (IP drift detection) and proactive reflattening
- Shadow publish: Publish flattened TXT at _spf.example.com; reference it via include in your live record to test without full cutover.
- Phased rollout: Start with p=none and pct=10–25 while monitoring, then increase pct and move to quarantine/reject.
- Rollback: Keep the previous TXT record version ready; if DMARC fail rate spikes, revert within minutes.
- Vendor coordination: Notify critical senders; validate their recommended SPF includes and DKIM selectors; get change windows.
AutoSPF supports:
- Shadow Publish and Split Traffic (pct-aware)
- One-click rollback with previous versions stored
- Vendor Watchlists with change notifications
Original data and case studies
- Aggregate before/after metrics (realistic example from a mid-size SaaS):
- Before flattening: SPF lookup median 8, p95 temperror 0.08%, DMARC fail 0.6% (mostly forwarding)
- After AutoSPF: Lookup median 1, p95 temperror 0.01%, DMARC fail 0.35% (DKIM primary pass path)
- TTL policy: 600s for 72h post-cutover, then 3600s; zero incidents reported
- Hypothetical FinTechCo rollout (10 senders):
- Preflight found two hidden includes that would have exceeded 10 lookups on some receivers
- AutoSPF consolidated vendor ranges from 48 to 21 CIDRs, final TXT split into 3 strings
- A/B strict alignment test showed one ESP signing with d=sub.vendor.com; FinTechCo added a second DKIM identity aligned to example.com; DMARC strict pass rose from 82% to 99.2%
- Rollback not needed; RUA alerts caught a transient DKIM selector outage at the marketing ESP within 30 minutes
FAQs
Will flattening break DMARC alignment?
No, flattening doesn’t change the MailFrom or header From domains; however, misconfigurations (e.g., unsafe redirects, omitted includes, or permerrors from oversized records) can cause SPF to fail, which may cause DMARC to fail if DKIM also fails. AutoSPF prevents these pitfalls with preflight linting and alignment simulation.
Do I still need DKIM if SPF is perfectly flattened?
Yes. DKIM is the most reliable DMARC pass path, especially through forwarding and mailing lists. A robust DKIM setup ensures DMARC passes even if SPF experiences transient DNS issues. AutoSPF’s DKIM Health checks and selector rotation tooling reinforce this.
How often should I reflatten?
Any time a vendor’s IP ranges change or you onboard a new sender—practically, daily or weekly checks. AutoSPF watches vendor ranges continuously and re-publishes within your TTL window.
Can flattening fix deliverability problems from forwarding?
No. Forwarding often breaks SPF but not DKIM. Ensure DKIM is aligned and robust (relaxed canonicalization) for forwarded mail. AutoSPF verifies DKIM resilience in its Mail Flow Emulator.
What if my DNS provider has small TXT limits?
Split strings and size-test before publish; if limits are too restrictive, consider redirect-based designs or multiple helper records. AutoSPF’s Size Guard and multi-string composer handle this automatically.
Conclusion: A practical, low-risk path with AutoSPF
To test an SPF flattener’s compatibility with DMARC and DKIM, you need a deliberate process: stage the flattened record, verify lookup budgets and syntax, exercise every sender with DKIM aligned, simulate relaxed/strict DMARC alignment, and watch your metrics before and after a controlled cutover with rollback.
AutoSPF operationalizes that process: it preflights your record, prevents lookup/size pitfalls, preserves macro semantics, simulates DMARC outcomes, validates DKIM selectors and alignment, and provides monitoring and one-click rollback. With AutoSPF, you can deploy flattening confidently, maintain DMARC compliance, and keep deliverability steady—even as your sender landscape evolves.
