Email Authentication Guide
By Abdul Sami, GTM Engineer · 7 min read · Last updated
Email authentication is three DNS layers working as one system: SPF lists who may send, DKIM proves the message is untampered, and DMARC enforces alignment plus policy. Since the 2024 Google/Yahoo bulk-sender rules, all three are mandatory for anyone sending at volume.
The three layers
| Layer | Proves | Weakness |
|---|---|---|
| SPF | The server may send for the domain | Breaks on forwarding; checks envelope, not From: |
| DKIM | Message signed by the domain, unaltered | Says nothing about who's authorized by itself |
| DMARC | From: domain aligns with SPF/DKIM + policy | Only as strong as the enforcement level |
Alignment: the concept that ties it together
A message can pass SPF with a random bounce domain and still spoof your From:. DMARC's alignment rule closes that: the domain SPF/DKIM authenticated must match the visible From: domain. This is why 'we have SPF' is not authentication.
Complete setup checklist
- SPF: one record, all senders included, -all.
- DKIM: 2048-bit, published and signing enabled per platform.
- DMARC: rua reporting on; p=quarantine, then p=reject.
- Every sending tool covered sequencer, transactional, helpdesk, CRM.
- Verified with the Domain Health Checker after any change.
FAQ
- What is the difference between SPF, DKIM and DMARC?
- SPF authorizes servers, DKIM signs messages, DMARC requires alignment and sets the failure policy. Each covers the others' gaps deploy all three.
- Do I need authentication for low-volume sending?
- Yes. Volume thresholds don't exempt you; unauthenticated mail from any sender gets filtered aggressively, and your domain stays spoofable without DMARC.
- What breaks authentication most often?
- New sending tools added without DNS records, platform migrations that disable DKIM signing, and duplicate SPF records created when adding a sender.
Abdul Sami
GTM Engineer, DFY GTM Systems
7+ years building outbound systems for B2B companies across the US, UK, Australia, and DACH. Specializes in Clay workflows, cold email infrastructure, intent-based lead sourcing, and deliverability.
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