SPF, DKIM, and DMARC

Why these three records exist

Mailbox providers receive a message and need to answer two questions:

1. Did this message actually come from a server that's allowed to send for this domain?
2. Was the message changed in flight?

SPF answers the first. DKIM answers both. DMARC ties the answers to the visible "From" address and tells the receiver what to do if anything is off.

SPF: Sender Policy Framework

SPF is a TXT record on your sending domain that lists the servers and services allowed to send mail "on behalf" of you. Receivers check the SMTP envelope sender ("MAIL FROM") domain against your SPF record.

An example for a domain that sends through Google Workspace and Warmbly:

v=spf1 include:_spf.google.com include:spf.warmbly.com ~all

• v=spf1 is the version.
• include: defers to another SPF record (Google's, ours).
• ~all softfails anything not listed. Receivers downgrade reputation but may still deliver.

DKIM: DomainKeys Identified Mail

DKIM signs outgoing mail with a cryptographic signature. The signature lives in a header; the public key lives in DNS under a "selector". When a receiver gets your message, it fetches your public key, verifies the signature, and confirms the message wasn't altered in flight.

A typical Google Workspace DKIM record looks like:

google._domainkey.acme.com TXT "v=DKIM1; k=rsa; p=MIIBIjANBgkqh..."

The google._ part is the selector. You'll have one per sending service, so different providers don't trample each other's keys. Use 2048-bit keys when the provider supports them; 1024 is the minimum.

DMARC: the policy that ties it together

DMARC sits on top of SPF and DKIM. It does three things:

1. Tells receivers what to do if a message fails SPF and DKIM (none / quarantine / reject).
2. Requires the authenticated domain to align with the visible From: domain.
3. Lets you collect aggregate reports so you can see who is sending as you.

A starter DMARC record at _dmarc.acme.com:

v=DMARC1; p=none; rua=mailto:[email protected]; ruf=mailto:[email protected]; pct=100; aspf=r; adkim=r

• p=none means monitor only. Start here.
• rua is the aggregate report mailbox.
• ruf is the forensic report mailbox.
• aspf=r and adkim=r set relaxed alignment; s is strict.

Alignment in plain English

"Alignment" means the domain that passed SPF (or signed with DKIM) must match the domain in the From: header.

• Strict SPF alignment: the MAIL FROM domain must equal the From: domain.
• Relaxed SPF alignment: the organisational domain must match. bounce.outreach.acme.com aligns relaxed with [email protected].
• Strict DKIM alignment: the DKIM d= domain must equal the From: domain.
• Relaxed DKIM alignment: the organisational domain must match.

Most senders should use relaxed alignment. Strict only matters when you're trying to prevent specific sub-domain spoofing.

The right way to roll out DMARC

1. Publish SPF and DKIM for every sending service. Verify with mail-tester.com or your provider's tool.
2. Publish DMARC with p=none; pct=100; rua=mailto:.... Don't change anything else for two weeks.
3. Read the aggregate reports. You'll discover services sending as you that you forgot about (the payroll provider, the newsletter from 2019).
4. Bring every legitimate sender into SPF and DKIM alignment.
5. Move to p=quarantine; pct=25. Ramp pct to 100 over a month while watching reports.
6. When clean, move to p=reject. This is the strongest spoof protection.

Common mistakes

• Multiple SPF records on the same domain: receivers treat that as PermError. There must be exactly one SPF record.
• Forgetting include: for a new sending service. Mail still leaves; reputation slowly tanks.
• 1024-bit DKIM keys never rotated. Mailbox providers are starting to penalise stale keys.
• DMARC p=reject on a domain with services you forgot about, which makes legitimate mail start disappearing.
• Cold mail from your transactional domain. Reputation issues bleed across.

Recommended sub-domain pattern

For a company at acme.com:

• acme.com carries transactional and human mail. DMARC reject after rollout.
• outreach.acme.com carries cold campaigns, with its own SPF, DKIM, and DMARC.
• news.acme.com carries newsletters, with separate authentication and reputation.

The sub-domain pattern keeps cold reputation issues from contaminating the inboxes you use to run the company. It also lets you change cold-email providers without disturbing transactional mail.