4 ms·

Seems to be just Fastmail domains but there might be some setup involved for custom domains. See their docs https://www.fastmail.com/help/technical/senderauthe

• eggsampler 8y ago
  I have received a spoofed email from my own domain, so I believe it's not just Fastmail domains but any custom domain on an account.
  • Riverheart 8y ago
    Do you host your custom domain with Fastmail? I may have misread but the article seemed to suggest you could potentially setup dmarc via the DNS page or whoever you host with.
• bscphil 8y ago
  If I understand this correctly, you need to be managing your own DNS (not letting Fastmail do it), and you probably need to set p=reject in your DMARC so that non-fastmail servers can't spoof your addresses. And if Fastmail allows Fastmail user A to spoof Fastmail user B, then the above still only protects you against non-Fastmail customers.