DMARC (RFC 7489) tells receiving mail servers what to do when a message claiming to be from your domain fails SPF or DKIM. You publish exactly one DMARC record at `_dmarc.<your-domain>`, regardless of which ESP you use — DMARC is a domain-level policy, not a per-sender configuration. Fastmail doesn't run DMARC for you, but their SPF + DKIM setup is what makes your DMARC checks pass.
Start every domain at `p=none` with a `rua` (aggregate report) address pointing somewhere you actually read. Watch the reports for two to four weeks to confirm 100% of legitimate mail is aligned, then progress to `p=quarantine` and finally `p=reject`. Skipping the monitoring step is the single most common way founders accidentally block their own mail.
Publish these DNS records
Add the following record(s) to your domain's DNS zone. Most registrars (Cloudflare, Route 53, Namecheap, GoDaddy) accept values exactly as shown.
- Type
TXT- Host
_dmarc- Value
v=DMARC1; p=none; rua=mailto:[email protected]
Where in Fastmail
The DMARC configuration lives in Fastmail → Settings → Domains → <your-domain> → Authentication.
Verify the records
Once published, run the DMARC Checker on your apex domain to confirm the record parses, reporting URIs are valid, and the policy is what you intended.
dig +short TXT _dmarc.your-domain.comCommon pitfalls
- Fastmail expects you to publish their MX records as well; without MX pointing at `in1-smtp.messagingengine.com` and `in2-smtp.messagingengine.com`, inbound mail won't reach the mailbox even if SPF/DKIM/DMARC are fine.
- Fastmail's masked-email feature uses a different sending IP pool. Outbound from masked addresses still aligns with your domain DKIM if you've configured custom domain sending.