SPF, DKIM & DMARC for Fastmail

SPF & DKIM — get both from Fastmail's own setup page

Fastmail's domain setup wizard (Settings → Domains → add or select your domain) gives you the exact DNS records to paste in one block, including SPF and three DKIM records (commonly selectors fm1, fm2, fm3, used so Fastmail can rotate keys without you touching DNS again). Copy those exactly as shown rather than from a guide — the DKIM values are unique to your account and Fastmail signs your mail with its own key, not one you generate yourself. A key pair from our generator would never actually get used to sign anything.

DMARC

Not specific to Fastmail — the same record works everywhere. Use the DMARC record generator to build yours.