Deliverability Glossary

Short working definitions of terms used across the KB. Each entry links to the article that treats the topic in depth. Terms are deliverability-focused; email design/copywriting vocabulary is out of scope.

A

Acceptance rate (delivery rate) — The percentage of sent messages accepted by receiving servers (SMTP 250). Acceptance is not inbox placement: an accepted message may still be spam-foldered. See Reputation Monitoring.

Allowlist / safelist — A recipient-side list of senders (addresses, domains, or IPs) whose mail bypasses some filtering. The opposite of a blocklist. Asking recipients to safelist your sending domain is a stopgap for critical mail, not a deliverability strategy; formal ISP allowlisting programs are largely historical (the CSA certification is a surviving example — see GMX/WEB.DE).

Apple Mail Privacy Protection (MPP) — Apple Mail feature that pre-fetches images through a proxy, firing open-tracking pixels without a human reading the message and hiding recipient IPs. A primary reason open rates are inflated and clicks should be weighted instead. See Metrics and Benchmarks.

ARC (Authenticated Received Chain) — RFC 8617 protocol that lets intermediaries (forwarders, mailing lists) preserve upstream authentication results so mail that legitimately breaks SPF/DKIM in transit can still be evaluated. See ARC.

ARF (Abuse Reporting Format) — RFC 5965 message format used by complaint feedback loops to report a recipient's "this is spam" action back to the sender. See Complaint Feedback Loops.

Authentication-Results header — RFC 8601 header added by the receiver recording its SPF/DKIM/DMARC (and related) verdicts for a message. The place to read how a receiver actually judged your authentication. See Authentication-Results Header.

B

Bayesian filter — A spam filter that classifies messages statistically from token frequencies learned from known spam and ham, rather than fixed rules. One of many content-analysis layers modern providers run. See Content and Design for Deliverability.

BIMI (Brand Indicators for Message Identification) — Standard for displaying a brand logo beside authenticated messages. Requires DMARC at enforcement (quarantine or reject) and, at most providers, a Verified Mark Certificate. See BIMI.

Blocklist (DNSBL / RBL) — A DNS-queryable list of IPs or domains suspected of sending spam. Receivers consume blocklist data as one input alongside internal metrics; Spamhaus carries the most weight. See Blocklists & Spamhaus.

Bounce — A delivery failure returned to the sender. Hard bounces (5.X.X) are permanent — invalid address, policy rejection; soft bounces (4.X.X) are temporary — full mailbox, deferral. See Metrics and Benchmarks and Enhanced Status Codes.

Bounce rate — Bounced messages as a percentage of sent. Common operating targets: under 2% healthy, investigate above 5%, never exceed 10%. See the threshold table.

Bulk sender — A sender exceeding a provider's volume threshold (Gmail: ~5,000 messages/day to Gmail), triggering stricter requirements — authentication, one-click unsubscribe, complaint-rate ceilings. See Gmail Sender Requirements.

C

CAN-SPAM — US law (2003) governing commercial email: truthful headers/subjects, physical address, functioning opt-out honored within 10 business days. Opt-out based, unlike CASL/GDPR. See CAN-SPAM.

CASL — Canada's Anti-Spam Legislation: requires express or implied consent before sending commercial electronic messages, plus identification and unsubscribe requirements. See CASL.

Complaint (spam report) — A recipient marking a message as spam. The single most damaging reputation signal; reported back to senders via feedback loops at some providers. See Complaint Feedback Loops.

Complaint rate — Complaints as a percentage of delivered mail. Target below 0.1%; Gmail/Yahoo enforce a 0.3% ceiling that must never be reached. See Metrics and Benchmarks.

Consent (express vs. implied) — Express consent is an explicit, recorded opt-in; implied consent is inferred from an existing business relationship. Which one suffices, and for how long, is jurisdiction-specific (CASL time-limits implied consent). See compliance articles.

CSA (Certified Senders Alliance) — German sender-certification program whose allowlist is honored by GMX/WEB.DE and other European receivers. See GMX/WEB.DE/mail.com.

D

DANE for SMTP — DNSSEC-based mechanism (TLSA records) for authenticating a receiving server's TLS certificate, closing the downgrade gap in opportunistic STARTTLS. See DANE.

Dedicated IP — A sending IP used by a single sender, giving full control over (and full responsibility for) its reputation. Only worthwhile above a sustained volume floor; low-volume senders keep a dedicated IP too "cold" to build reputation. See Basic IP Allocation.

Deferral (greylisting/throttling response) — A temporary 4XX refusal instructing the sender to retry later. Rising deferrals at one provider are often throttling — the step before blocking. See Reputation Monitoring.

Delivery vs. deliverability — Delivery: the receiving server accepted the message. Deliverability: where it landed afterward — inbox, tab, spam, or silently dropped. See Reputation Monitoring.

DKIM (DomainKeys Identified Mail) — RFC 6376 cryptographic signature over selected headers and the body, verified via a public key in DNS; proves domain responsibility and message integrity in transit. See DKIM.

DMARC — Domain-based Message Authentication, Reporting and Conformance: publishes a policy (none/quarantine/reject) for mail failing aligned SPF or DKIM, plus reporting. See DMARC and DMARC Deployment.

DMARC alignment — The requirement that the domain in the visible From header match (exactly or organizationally) the domain SPF or DKIM validated. Authentication without alignment does not pass DMARC. See DMARC.

Double opt-in (confirmed opt-in) — Signup flow where the address is not mailed until the subscriber clicks a confirmation link. Eliminates typo bounces, blocks malicious sign-ups, and proves consent. See List Hygiene.

DSN (Delivery Status Notification) — RFC 3464 machine-readable bounce message reporting delivery failure, delay, or success, carrying per-recipient status codes. See Delivery Status Notifications.

E

Email harvesting (scraping) — Collecting addresses without consent, typically by crawling websites. Harvested lists are rich in spam traps and invalid addresses; never mail them. See List Hygiene.

Email validation — Pre-send verification that an address is syntactically valid, has working MX records, and (via verification services) an existing mailbox. Done at signup, it prevents hard bounces later. See List Hygiene.

Engagement — Recipient interactions providers score senders on: opens, clicks, reading time, replies, deletes-without-reading, moves out of spam. The statistical basis for the "wanted and expected" judgment. See Foundations.

Enhanced status codes — RFC 3463 three-part codes (e.g., 5.1.1 = bad destination mailbox) that make SMTP replies machine-parseable for bounce classification. See Enhanced Status Codes.

Envelope sender (MAIL FROM / Return-Path / 5321.MailFrom) — The address given in the SMTP MAIL FROM command, where bounces are sent; distinct from the visible From header. SPF is evaluated against this domain. See SPF and SMTP.

F

False positive — A legitimate message wrongly classified as spam. The cost providers weigh against missed spam; the reason engagement history can rescue borderline mail.

Feedback loop (FBL) — Provider mechanism reporting recipient spam complaints back to the sender (usually in ARF format) so complainers can be suppressed immediately. See Complaint Feedback Loops.

FCrDNS (forward-confirmed reverse DNS) — The sending IP's PTR record resolves to a hostname that resolves back to the same IP. A baseline infrastructure requirement at most major providers. See Sending Infrastructure Practices.

Feedback-ID header — Gmail's FBL mechanism: a sender-supplied header whose identifiers Gmail aggregates into per-campaign spam-rate statistics in Postmaster Tools (individual complainers are not revealed). See Google Postmaster Tools.

From header (5322.From) — The author address displayed to recipients, defined by RFC 5322. DMARC alignment is anchored on this domain. See Message Format.

G

GDPR — EU data-protection regulation; for email marketing it requires a lawful basis (usually explicit consent), data-subject rights, and easy withdrawal. Broader than an anti-spam law. See compliance overview.

Google Postmaster Tools — Google's free sender dashboard: domain/IP reputation, spam-rate, authentication pass rates, and delivery errors for mail to Gmail. See Google Postmaster Tools.

Greylisting — Receiver technique that temporarily rejects mail from unknown sources, accepting it on retry; exploits the fact that much spamware doesn't retry. Appears to senders as deferrals. See SMTP.

H

Hard bounce — Permanent delivery failure (5.X.X): nonexistent address or domain, policy rejection. Remove the address immediately and never re-send. See List Hygiene.

Honeypot — A decoy — either a pristine spam-trap address planted to catch list harvesters, or a hidden form field that only bots fill in, used to block bot signups. See Spam Traps.

I

Inbox placement rate (IPR) — The percentage of delivered mail that reaches the inbox rather than spam, usually measured per provider via seed testing. See Reputation Monitoring.

IP warming (warm-up) — Gradually ramping volume on a new IP (or domain) so receivers can build a reputation history; sudden full-volume sending from a cold IP triggers filtering. See IP Warm-Up.

L

List churn — The rate at which addresses leave a list through unsubscribes, bounces, and abandonment. Marketing databases degrade by roughly 22% per year, so hygiene is continuous, not a one-off. See Reputation Monitoring.

List hygiene — The ongoing practice of removing invalid, bouncing, complaining, and unengaged addresses to keep the metrics providers score. See List Hygiene and Sunset Policies.

List-Unsubscribe header — RFC 2369 header advertising unsubscribe endpoints (mailto/HTTPS) that mail clients surface as a native unsubscribe control. See List-Unsubscribe & One-Click.

M

Mailbox provider (MBP / ISP) — The organization operating recipients' mailboxes (Gmail, Yahoo, Microsoft, corporate servers). This KB uses "mailbox provider" and "ISP" interchangeably for the receiving side.

MTA / MSA / MDA / MUA — The mail-handling roles: Mail Transfer Agent (relays between servers), Mail Submission Agent (accepts mail from clients), Mail Delivery Agent (files into the mailbox), Mail User Agent (the recipient's client). See Email Architecture.

MTA-STS — Policy mechanism (HTTPS-published) by which a receiving domain requires authenticated TLS for inbound SMTP, defeating STARTTLS downgrade attacks without DNSSEC. See MTA-STS.

MX record — DNS record naming the servers that receive mail for a domain. Checked during address validation and required (or an A record fallback) for a domain to receive bounces.

O

One-click unsubscribe — RFC 8058: a List-Unsubscribe HTTPS URI plus List-Unsubscribe-Post header enabling unsubscribe with a single POST, no landing page. Mandatory for bulk senders under the 2024 Gmail/Yahoo requirements. See List-Unsubscribe & One-Click.

Open rate — The percentage of delivered messages whose tracking pixel fired. Inflated by Apple MPP and image prefetching; treat as approximate and weight clicks/conversions instead. See Metrics and Benchmarks.

Opt-in / opt-out — Opt-in: the recipient explicitly requested the mail (the consent model of CASL/GDPR). Opt-out: mail may be sent until the recipient unsubscribes (the CAN-SPAM model). See compliance articles.

P

PECR — UK Privacy and Electronic Communications Regulations: consent-based rules for UK electronic marketing, alongside UK GDPR. See UK PECR.

Postmaster — The role responsible for a domain's mail system; postmaster@ is a reserved address that must accept mail. Provider "postmaster pages" publish their sender requirements — see the provider articles.

Preference center — A page where subscribers adjust frequency, topics, and channels instead of fully unsubscribing — converting would-be complaints into retained, lower-frequency subscriptions. See List Hygiene.

Q

Quarantine — In DMARC, the policy requesting that failing mail be treated with suspicion (typically spam-foldered) rather than rejected. More generally, filter-side holding of suspect mail. See DMARC.

R

Re-engagement (win-back) campaign — A last-chance message to inactive subscribers asking them to confirm continued interest before they are purged under a sunset policy. See List Hygiene.

Reverse DNS (PTR record) — DNS mapping from an IP back to a hostname. Sending IPs need a non-generic PTR that forward-confirms (see FCrDNS); many providers reject mail from IPs without one. See Sending Infrastructure Practices.

S

Seed list — A panel of monitored test mailboxes across providers used to measure where a given message lands (inbox/tab/spam/missing). Seed results skew conservative because seeds have no engagement history. See Reputation Monitoring.

Sender reputation — The receiver-side score(s) attached to your sending IPs and domains, computed from complaints, bounces, engagement, spam-trap hits, volume patterns, and authentication. Determines inboxing, throttling, and blocking. See Reputation Monitoring.

Shared IP — A sending IP whose reputation is pooled across many senders. Appropriate below the dedicated-IP volume floor; the trade-off is exposure to neighbors' behavior. See Basic IP Allocation.

SMTP — Simple Mail Transfer Protocol (RFC 5321), the server-to-server transport for email; its reply codes are the raw material of bounce handling. See SMTP digest.

SNDS (Smart Network Data Services) — Microsoft's per-IP sender dashboard for mail to its consumer domains: volume, filter verdict, complaint rate, trap hits. Paired with JMRP, its complaint feedback loop. See Microsoft SNDS & JMRP.

Soft bounce — Temporary delivery failure (4.X.X): full mailbox, server unavailable, throttling. Retried automatically; suppress addresses that soft-bounce persistently. See Metrics and Benchmarks.

Spam trap — An address that generates reputation damage when mailed: pristine traps never belonged to a person (reachable only via bought/scraped lists); recycled traps are abandoned real addresses reactivated as traps; typo traps catch sloppy signup handling. See Spam Traps.

SpamAssassin — Widely deployed open-source content filter that scores messages against rule sets; its scores appear in some receivers' headers and in seed-test reports. See Deliverability Testing Tools.

SPF (Sender Policy Framework) — RFC 7208 DNS record listing the hosts authorized to send mail for a domain, evaluated against the envelope sender. Subject to a 10-DNS-lookup limit and broken by forwarding. See SPF.

Spoofing — Forging sender identity (usually the From header) to impersonate a trusted party. The threat SPF, DKIM, and DMARC exist to counter. See DMARC.

Sunset policy — Rules that reduce frequency to, then suppress, addresses that stop engaging — before they decay into hard bounces or recycled spam traps (~12 months of inactivity). See List Hygiene and Sunset Policies.

Suppression list — The do-not-mail list of unsubscribed, hard-bounced, and complaining addresses. Suppressions are a reputation defense; never bypass them. See Reputation Monitoring.

T

Throttling — A receiver limiting how much mail it accepts from a sender per unit time, surfacing as deferrals. Also: sender-side rate limiting to stay under those limits. See Metrics and Benchmarks.

TLS-RPT — Reporting channel (RFC 8460) through which receivers tell a domain about TLS negotiation failures for its inbound mail, complementing MTA-STS/DANE. See TLS-RPT.

Tracking pixel — An invisible remote image whose fetch registers an "open." The mechanism behind open metrics and their proxy-driven unreliability. See Metrics and Benchmarks.

Transactional email — Mail triggered by a recipient's own action (receipt, password reset, notification). Generally exempt from marketing consent rules and from marketing suppression — but not from deliverability: it should be isolated from marketing streams. See Advanced IP Segmentation.

U

Unsolicited commercial email (UCE) — Commercial mail sent without recipient permission — spam, in the operational and often legal sense.

Unsubscribe rate — Unsubscribes as a percentage of delivered mail (healthy: under ~0.3%). An unsubscribe is a far cheaper negative signal than a complaint — make it easy. See Metrics and Benchmarks.

V

VMC (Verified Mark Certificate) — Certificate attesting ownership of a (usually trademarked) logo, required by Gmail and others for BIMI logo display. See BIMI.

Related

• Deliverability Testing Tools — tools that operationalize many of these concepts
• Foundations of Email Deliverability — the principle the vocabulary serves