SPF, DKIM and DMARC setup Claude skill

Use this when a domain sends email and authentication is missing, broken or failing alignment. Without all three records passing, major mailbox providers junk or reject bulk and cold mail.

Inputs

• The domain.
• Every service that sends email as that domain: mailbox provider, marketing platform, CRM, support desk, billing, product notifications.
• Current DNS records, if any.

The play

1. Inventory senders

List every service and how it sends: does it send "as" your domain, and does it support custom DKIM? Missing a sender here is how a DMARC rollout ends up blocking invoices.

2. SPF

One TXT record on the domain listing every service, ending in ~all (soft fail) or -all (hard fail).

v=spf1 include:<mail provider> include:<marketing service> ~all

Rules: exactly one SPF record per domain; 10 DNS lookups or fewer (each include counts, and nested includes count too). Over 10 lookups causes a permanent error and SPF fails for everyone. Use subdomains for high-volume services to stay under the limit.

3. DKIM

Turn on signing in each service and publish the key it gives you (usually a CNAME or TXT at <selector>._domainkey.<domain>). Then confirm signing is actually enabled; a published key with signing off is a common silent failure.

4. DMARC

Start in monitoring mode and tighten over weeks:

Read references/dns-records.md for record examples, alignment explained and how to read aggregate reports.

5. Verify

Send test emails to a mailbox you control and check the headers: spf=pass, dkim=pass, dmarc=pass, with the From domain aligned.

What good looks like

• Every legitimate sender passes DMARC before the policy moves past p=none.
• SPF stays under 10 lookups with room to add a service.
• DMARC reports go to an address the user owns and someone actually reads them.
• The common mistake is publishing p=reject on day one and silently blocking the billing system’s emails.

Rules

• MUST inventory all senders before changing SPF or DMARC.
• MUST start DMARC at p=none.
• MUST send DMARC reports only to addresses or services the user controls.
• NEVER publish a second SPF record; merge into one.
• NEVER change DNS without the user’s explicit approval.

Example prompts

• "Write SPF, DKIM and DMARC for acme.com. We send from our mailbox provider, a marketing platform and a support desk."
• "Our DMARC report shows failures from an IP we don’t recognise. What is it?"
• "We’re over the SPF lookup limit. Fix it."

Record examples

Host names and include values above are placeholders; use the exact values each service documents.

Alignment in plain words

DMARC passes when at least one of these is true:

• SPF aligned: the message passes SPF and the envelope sender domain (Return-Path) matches the visible From domain.
• DKIM aligned: the message has a valid DKIM signature whose d= domain matches the visible From domain.

Many services send with their own Return-Path, so SPF alignment fails even though SPF passes. That is why custom DKIM for each service matters most: it is usually the route to alignment.

Relaxed alignment (the default) accepts subdomains: news.yourdomain.com aligns with yourdomain.com.

SPF lookup counting

Each of these costs one lookup: include, a, mx, ptr, exists, redirect. Nested includes inside an include also count. ip4 and ip6 cost nothing.

Ways to stay under 10:

• Move a high-volume service to a subdomain with its own SPF.
• Remove services you no longer use.
• Replace an include with its ip4 ranges only if the service publishes stable ranges (otherwise this breaks when they change).

Reading aggregate (rua) reports

Group rows by source IP and sending service:

Move to quarantine only when every legitimate source is aligned.