Article URL: https://senderledger.com/articles/what-dmarc-actually-protects-you-from Comments URL: https://news.ycombinator.com/item?id=49153361 Points: 16 # Comments: 4

DMARC gets asked to do a lot of jobs it was never designed for. Teams reach for it as a spam filter, a phishing filter, and a general trust signal. It is none of those. The current DMARC protocol, defined in RFC 9989, answers a deliberately narrow question: did the owner of the domain in the visible From address authorise this message, and can that authorisation be established through an aligned SPF or DKIM result? That question is worth answering. It is also much narrower than the reputation DMARC has picked up. Getting the boundary right matters, because a team that reaches p=reject believing they are now phishing-proof will skip the controls that cover everything DMARC leaves untouched. Three terms come up throughout, so here they are in plain English. SPF is a published list of the servers a domain says are allowed to send mail for it; a receiver checks whether the message actually came from one of them. DKIM is a cryptographic signature added to the message, so a receiver can confirm it genuinely came from the signing domain and was not tampered with in transit. DMARC ties both back to one thing: the visible From address. That last term matters more than it sounds. Every email carries two "from" addresses. One is the visible From, the name and address your mail app shows you (for example, "Your Bank <alerts@your-bank.com>"); it is the part a human actually reads and trusts. The other is a hidden envelope address used behind the scenes for delivery, which you never see. An attacker can set these two to different values, which is exactly how a message can look like it is from your bank while being delivered on behalf of somewhere else. DMARC's job is to make sure authentication lines up with the address you can see, not the hidden one. All three live as text records in your domain's DNS, the same place your website's address is configured. You do not need to memorise the syntax; it helps to recognise the shape. An SPF record lists who is allowed to send. This one authorises Google Workspace and a marketing tool, and says anything else should be treated as suspicious: The include: entries pull in each provider's own list of servers, and -all means "if it is not on those lists, it is not us". A DKIM record publishes the public half of the signing key, so receivers can check the signature on your mail. The long string is the key itself: