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Deliverability

DMARC

DMARC ties SPF and DKIM together: it tells receiving servers what to do when a message claiming to be from your domain fails those checks, and asks them to report back on what they saw.

Without it, SPF and DKIM produce signals nobody is obliged to act on. DMARC turns them into a policy — do nothing, quarantine, or reject — and gives you the reports showing which servers are sending as your domain, including the ones you did not know about.

Start at p=none, which changes nothing about delivery but turns reporting on. Read the reports for a few weeks, confirm every legitimate sender passes, and only then tighten to quarantine and reject. Jumping straight to reject on an unaudited sending stack is how a company discovers its invoicing system was never authenticated.

Goes deeper: Why Your Cold Emails Land in Spam

Related terms

SPF
SPF is a DNS record listing which servers may send email for your domain. Receiving servers check it to see whether a message claiming to come from you actually came from a server you authorised.
DKIM
DKIM adds a cryptographic signature to every message you send, which the receiving server checks against a public key in your DNS. It proves the message came from your domain and was not altered on the way.
Deliverability
Deliverability is whether your email reaches the inbox rather than the spam folder or nowhere at all. It is decided by sending reputation, authentication and list quality — not by your copy.
Sender reputation
Sender reputation is the score mailbox providers keep on your domain and sending IP, based on how recipients react to your mail. It decides whether your next message reaches the inbox.