Email deliverability glossary

SPF record

Sender Policy Framework

An SPF record is a DNS TXT record listing which servers are allowed to send mail for your domain. Receiving servers check the envelope sender against it to detect forgery.

DKIM

DomainKeys Identified Mail

DKIM cryptographically signs outgoing email with a private key. Receivers fetch the matching public key from DNS and verify the signature, proving the message was authorised and not altered in transit.

DMARC

Domain-based Message Authentication, Reporting and Conformance

DMARC is a DNS policy that tells receivers what to do when a message fails SPF and DKIM alignment: nothing, quarantine it, or reject it. It also requests aggregate reports on authentication results.

MAIL FROM domain

Envelope sender, Return-Path

The MAIL FROM domain is the address in the SMTP envelope that receives bounces. Setting it to a subdomain you own — rather than your provider’s domain — is what makes SPF align for DMARC.

List-Unsubscribe

List-Unsubscribe is an email header giving mailbox providers a machine-readable way to opt someone out. It renders as the native unsubscribe link Gmail shows next to the sender name.

One-click unsubscribe

RFC 8058

RFC 8058 one-click unsubscribe lets a mailbox provider opt a recipient out with a single POST request, no landing page and no confirmation step. Gmail and Yahoo require it from bulk senders.

Email warmup

Mailbox warming

Email warmup gradually increases sending from a new mailbox while generating positive engagement, so mailbox providers build a reputation for the address before it sends cold volume.

Reply capture

Reply capture is the inbound routing that delivers replies back into your sending platform. Without it, warmup cannot be verified and genuine responses from prospects are lost.

Inbox placement

Deliverability rate

Inbox placement is the share of delivered messages that reach the primary inbox rather than spam or a promotions tab. It is distinct from delivery rate, which only counts acceptance by the server.

Sending reputation

Domain reputation

Sending reputation is the trust score mailbox providers assign to your domain and IP based on complaint rate, bounce rate, engagement and authentication history. It determines inbox placement.

Hard bounce

A hard bounce is a permanent delivery failure — the address does not exist or the domain rejects it outright. Hard-bounced addresses must be suppressed immediately and never retried.

Spam trap

Honeypot address

A spam trap is an email address that exists only to catch senders using poor list practices. Mail to one is strong evidence of purchased or scraped data, and can get a domain blocklisted.

Spam complaint rate

Spam complaint rate is the share of delivered messages recipients mark as spam. Gmail and Yahoo require bulk senders to stay below 0.3%, with 0.1% the practical working target.