Email deliverability measures whether an email actually lands in a recipient's inbox—distinct from simple delivery (where an SMTP server accepts the message). Receiving mail servers use authentication, sender reputation, and content signals to decide inbox placement.
Authentication is foundational. SPF authorizes specific IP addresses to send mail for a domain via DNS TXT record. DKIM cryptographically signs message content with a private key, publishing the public key in DNS so receivers verify the signature. DMARC is a policy layer: it requires SPF or DKIM alignment (the From: domain must match the authenticated sending domain), instructs receivers what to do on failure (quarantine or reject), and enables reporting.
Beyond authentication, mailbox providers weight sender IP/domain reputation history, spam complaint rates, bounce rates, user engagement (opens and clicks), list hygiene, and content analysis. Transport encryption via MTA-STS signals security compliance. Large providers like Gmail and Yahoo apply proprietary algorithms combining all these signals.
Concrete example: A company sends newsletters from [email protected]. Its DNS has an SPF record authorizing the ESP's sending IPs, a DKIM selector (selector1._domainkey.example.com), and a DMARC policy v=DMARC1; p=quarantine. When a spammer spoofs example.com without valid SPF/DKIM alignment, DMARC tells receivers to quarantine it, protecting the company's inbox placement for legitimate mail.
TipAuthentication alone doesn't guarantee inbox placement—sender reputation and engagement history matter equally. Start with SPF, DKIM, and DMARC, then monitor bounce rates and spam complaints to maintain long-term deliverability.