Email deliverability glossary, A to Z

Accept-all

A verification result for an address on a domain whose mail server accepts every recipient at the RCPT TO stage. The address may or may not have a real mailbox, so the result is a risk signal rather than proof of deliverability.

ARC

Authenticated Received Chain

A protocol (RFC 8617) that lets forwarders and mailing lists record the SPF, DKIM and DMARC results they saw and seal them with a signature. When forwarding later breaks SPF or DKIM, the final receiver can consult that chain before deciding how to treat the message.

Authentication-Results header

A header (RFC 8601) the receiving server adds to record the outcome of each authentication check, for example spf=pass, dkim=pass and dmarc=pass with the domains that were checked. It is the quickest place to see why a message passed or failed.

BIMI

Brand Indicators for Message Identification

A specification that lets a brand logo appear next to authenticated mail in supporting inboxes. The domain publishes a TXT record at default._bimi.<domain> pointing to an SVG logo, needs an enforced DMARC policy of quarantine or reject. Some providers, including Gmail, also require a mark certificate (VMC or CMC).

Blocklist

also called a blacklist or DNSBL

A list of IP addresses or domains associated with spam or abuse that receiving servers query in real time. A DNS-based blocklist (DNSBL, RFC 5782) is checked by reversing the IP octets and looking them up under the list zone; an answer such as 127.0.0.2 means the address is listed.

Bounce code

SMTP reply code

The three digit reply a mail server returns to each SMTP command (RFC 5321). The first digit gives the class: 2xx means success, 4xx a temporary failure worth retrying, and 5xx a permanent failure.

Bounce rate

The share of sent messages that were rejected by the receiving server, usually reported separately for hard and soft bounces. A hard bounce rate under 2% per send is the common ceiling, and a well kept list stays under 1%.

Bulk sender

Under Gmail rules, a sender that has sent close to 5,000 or more messages to personal Gmail accounts in one day; the status does not expire. Bulk senders must authenticate with SPF, DKIM and DMARC, support one-click unsubscribe and keep their spam rate low.

Catch-all domain

A domain configured to accept mail for any local part, including addresses with no mailbox behind them. Because RCPT TO returns 250 for every recipient, an SMTP check alone cannot confirm a specific address. Verifiers detect it by probing an address that cannot exist.

Complaint rate

also called spam rate

The share of delivered messages that recipients mark as spam. Gmail asks senders to keep the rate shown in Postmaster Tools below 0.1% and never reach 0.3%, and Yahoo uses the same 0.3% ceiling.

Deliverability

Whether an accepted message reaches the inbox rather than the spam folder. Delivery only means the receiving server accepted it; deliverability is decided after that by filtering based on reputation, authentication and engagement.

Disposable email address

A temporary inbox from a service such as Mailinator or Guerrilla Mail, used to receive one signup email and then abandoned or deleted automatically. The address may pass an SMTP check, but nobody will read later mail sent to it.

DKIM

DomainKeys Identified Mail

An authentication method (RFC 6376) in which the sending server signs chosen headers and the body with a private key. Receivers fetch the public key from a TXT record at <selector>._domainkey.<domain> and confirm the signature; the d= tag names the domain taking responsibility.

DMARC

Domain-based Message Authentication, Reporting, and Conformance

A policy published as a TXT record at _dmarc.<domain> that tells receivers how to handle mail failing authentication: p=none, quarantine or reject. A message passes when SPF or DKIM passes for a domain aligned with the From address. Now defined in RFC 9989, which replaced RFC 7489 in 2026.

DMARC alignment

identifier alignment

The DMARC requirement that the domain verified by SPF (the envelope sender) or by DKIM (the d= domain) matches the domain in the visible From header. Relaxed alignment accepts the same organizational domain, such as mail.example.com for example.com; strict alignment needs an exact match.

Domain warmup

Building reputation for a new sending domain by starting with low volume to the most engaged recipients and raising it over several weeks. Mailbox providers now weight domain reputation heavily, so a new domain is treated with caution even on an established IP.

Double opt-in

also called confirmed opt-in

A signup flow in which a new subscriber is added to the list only after clicking a link in a confirmation email. It proves the address works and that its owner asked to join, which keeps typos, bots and spam traps off the list.

DSN

Delivery Status Notification

The machine readable bounce message (RFC 3464) a mail server returns to the envelope sender when it cannot deliver a message. It carries the failed recipient, the SMTP reply and an enhanced status code, which is how sending platforms classify hard and soft bounces.

Email validation

Checks that an address is correctly formatted and that its domain is set up to receive mail, without contacting the mailbox. It catches typos and dead domains but cannot tell whether a specific mailbox exists.

Email verification

Checks whether a specific mailbox exists and accepts mail, usually through a live SMTP conversation with the recipient server that stops before any message is sent. Verification services add risk signals such as catch-all, disposable and role-based detection.

Engagement

Recipient actions that mailbox providers use as filtering signals, such as opening, replying, moving a message out of spam, deleting without reading or reporting spam. Open rates are a weak proxy because Apple Mail Privacy Protection loads images for many messages automatically.

Enhanced status code

A three part code in the form class.subject.detail (RFC 3463) that adds precision to an SMTP reply. In 5.1.1, the 5 means permanent failure, the 1 means an addressing problem and the final 1 means the mailbox does not exist.

Envelope sender

also called MAIL FROM or reverse path

The address given in the SMTP MAIL FROM command (RFC 5321), separate from the From header people see. Bounces are sent to it and SPF is checked against its domain.

Feedback loop

FBL

A program run by a mailbox provider that reports to the sender when a recipient marks a message as spam, typically in Abuse Reporting Format (RFC 5965). Gmail runs an aggregate version in Postmaster Tools that does not identify individual recipients.

Greylisting

An anti-spam technique where a server temporarily rejects mail from an unfamiliar combination of sending IP, sender and recipient with a 4xx reply, then accepts it when the sender retries after a delay. During verification it can delay or block a definitive answer.

Hard bounce

A permanent delivery failure signaled by a 5xx reply, most often because the mailbox or domain does not exist. The address should be suppressed at once and never retried.

Honeypot address

A spam trap placed where only automated scrapers will find it, such as hidden page markup. No person could have subscribed with it, so mail sent to it proves the sender harvested addresses.

Inbox placement

Where an accepted message actually lands: the primary inbox, a category tab such as Gmail Promotions, or the spam folder. It is measured with seed testing and with provider data such as Postmaster Tools.

IP warmup

Sending a gradually increasing daily volume from a new dedicated IP address so mailbox providers can build a reputation for it. Starting at full volume on a cold IP often triggers rate limiting with 4xx replies or spam folder placement.

List decay

The steady loss of valid addresses on a list as people change jobs, abandon mailboxes and let domains expire. A list that was clean when verified will hold stale addresses months later without any new signups.

List hygiene

The ongoing practice of removing invalid, risky and unengaged addresses from a mailing list: verifying new signups, suppressing bounces and complaints, and sunsetting inactive subscribers.

List-Unsubscribe header

A header (RFC 2369) that gives mail clients a mailto: address and/or an https: URL for unsubscribing, so they can show their own unsubscribe button. Gmail and Yahoo require it on bulk mail.

MTA-STS

SMTP MTA Strict Transport Security

A standard (RFC 8461) that lets a domain require TLS with a valid certificate for inbound mail. It is announced by a TXT record at _mta-sts.<domain> and a policy file served at https://mta-sts.<domain>/.well-known/mta-sts.txt in enforce, testing or none mode.

MX record

Mail Exchange record

A DNS record naming a server that accepts mail for a domain, with a preference number; senders try the lowest number first. When a domain has no MX record, RFC 5321 has senders fall back to its A or AAAA record.

Null MX

A single MX record with preference 0 and a target of "." (RFC 7505) that declares a domain accepts no email. Senders should not attempt delivery, so every address at the domain is undeliverable.

One-click unsubscribe

RFC 8058

An extension to List-Unsubscribe that adds the header List-Unsubscribe-Post: List-Unsubscribe=One-Click, letting the mail client unsubscribe the recipient with a single HTTPS POST and no further steps. Both headers must be covered by a valid DKIM signature, and Gmail expects requests honored within two days.

Plus addressing

also called subaddressing

Appending +tag to the local part of an address, as in user+news@example.com, so mail still reaches the base mailbox (RFC 5233 describes the convention). Support depends on the provider, so the tagged and untagged forms are separate entries on a list.

Pristine spam trap

An address created only to catch spam that has never belonged to a person or opted in to anything. Hitting one indicates a purchased, rented or scraped list.

PTR record

reverse DNS

A DNS record under in-addr.arpa or ip6.arpa that maps an IP address back to a hostname. Receivers expect a sending IP to have a PTR whose hostname resolves forward to the same IP, and Gmail requires it of all senders.

RCPT TO

The SMTP command that names one recipient of a message (RFC 5321). The server reply is what mailbox verification reads: 250 accepts the address, 550 with 5.1.1 rejects it as unknown, and a 4xx defers the decision.

Re-engagement campaign

A short series sent to subscribers who have stopped opening or clicking, asking them to confirm they still want the mail. Those who do not respond are removed under the sunset policy.

Recycled spam trap

A once real address that was abandoned, bounced as unknown for a period, and was then reactivated by the provider as a trap. Mail to it shows that a sender kept mailing without removing addresses that bounced.

Return-Path

The header the final receiving server adds to record the envelope sender address from MAIL FROM (RFC 5321). It shows where bounces go, and its domain is the one SPF evaluates and DMARC checks for alignment.

Role-based address

A mailbox tied to a function rather than a person, such as info@, sales@ or support@. It is often read by several people or routed to a ticketing system, so no single reader may have opted in.

Seed testing

Sending a campaign to a set of test mailboxes at different providers and checking whether each copy lands in the inbox, a tab or spam. It is an estimate, because filtering for real recipients is also personalized by their own engagement.

Sender reputation

The trust mailbox providers place in a sending domain and IP address, built from bounces, spam complaints, trap hits, engagement and authentication over time. It decides whether new mail is delivered to the inbox, filtered to spam or rejected.

SMTP handshake

The opening of an SMTP session: a TCP connection to port 25, the server 220 greeting, the client EHLO (or HELO), and the server reply listing its extensions. Verification continues with MAIL FROM and RCPT TO, then quits before DATA so no message is sent.

SMTP verification

also called mailbox verification

Checking whether a mailbox exists by connecting to the domain mail server, starting a normal delivery conversation and reading the reply to RCPT TO. The verifier disconnects before the DATA stage, so no email is sent.

Soft bounce

A temporary delivery failure signaled by a 4xx reply, for example a full mailbox, a busy server, rate limiting or greylisting. The address may still be valid, so the message is retried; repeated soft bounces over several sends are usually treated as hard.

Spam trap

An address controlled by a mailbox provider or blocklist operator to identify senders with poor list practices. It never signs up for anything, so mail it receives shows a list was bought, scraped, mistyped or never cleaned. The main types are pristine, recycled and typo traps.

SPF

Sender Policy Framework

A TXT record (RFC 7208) listing the servers allowed to send mail for a domain. Receivers check the connecting IP against the record for the envelope sender domain; evaluation may use at most 10 DNS lookups, and exceeding that is a permerror.

Sunset policy

A rule for removing subscribers who have not engaged for a set period, such as 6 or 12 months, usually after a re-engagement attempt. It keeps mail flowing to people who read it and lowers the chance of hitting recycled traps.

Suppression list

A list of addresses a sender must never mail again, such as hard bounces, spam complaints and unsubscribes. It is checked before every send and overrides any later import of the same address.

Syntax check

The first verification step, confirming an address follows the format rules of RFC 5321 and RFC 5322: a local part, one @ and a valid domain, with a local part of at most 64 octets. Passing says nothing about whether the mailbox exists.

TLS-RPT

SMTP TLS Reporting

A standard (RFC 8460) where a domain publishes a TXT record at _smtp._tls.<domain> with a rua= address. Sending servers then deliver daily reports of TLS connection failures, which show whether MTA-STS or DANE policies are working.

Toxic address

A vendor label, not a standard term, for an address that may accept mail but is likely to harm sender reputation: a suspected trap, a known complainer, a bot signup or an address tied to abuse. Most senders suppress these rather than mail them.

Typo domain

A misspelling of a common mail domain, such as gmial.com or hotmial.com. Some are unregistered and bounce, while others are registered and run as spam traps, so verification flags them and can suggest the intended domain.

Typo trap

A spam trap at a misspelled domain or address that collects mail from people who mistyped their own address at signup. Hits show that a sender is not confirming new addresses.

Unknown result

A verification outcome when the mail server gives no clear answer, for example after a timeout, greylisting or a blocked probe. RocketVerifier never charges credits for unknown results.

VRFY

An SMTP command (RFC 5321) that asks a server whether a mailbox exists. Most servers disable it or answer 252 without confirming, to stop address harvesting, so verifiers read the RCPT TO reply instead.