The deliverability glossary

This is the course's reference page: every term the lessons define, collected in one place and kept deliberately short. The big terms link to the lesson that teaches them properly; the definition here is the reminder, not the education. Bookmark it. Nobody remembers what a permerror is at the exact moment they need to.

A-D

• acceptance - the receiving server saying yes during the SMTP conversation. Acceptance is what "delivered" statistics count, and it says nothing about the folder.
• alignment - the DMARC requirement that the domain which passed SPF or DKIM matches the visible From domain. Taught in Authentication alignment.
• allowlist - a list of senders a receiver treats as trusted, bypassing some filtering. The opposite of a blocklist.
• apex domain - the bare registered domain (example.com), as opposed to a subdomain like send.example.com.
• Apple Mail Privacy Protection - the Apple Mail feature (since 2021) that prefetches messages through Apple's servers, recording opens the recipient never made. The reason open rates read high and only their trend carries signal.
• ARC - Authenticated Received Chain (RFC 8617): a way for forwarders to preserve the authentication results a message had before forwarding broke them. Covered in Beyond the big three.
• Authentication-Results - the header where the receiving provider records its authentication verdicts for a message. How to read it: Reading email headers.
• BIMI - Brand Indicators for Message Identification: a DNS record that lets your logo appear beside your messages in some inboxes. It requires an enforcing DMARC policy first.
• blocklist - a published list of IP addresses or domains associated with spam, consulted by receivers during filtering. Taught in Blocklists explained.
• bounce - a message returned as undeliverable. See hard bounce and soft bounce.
• bounce domain - the domain in the Return-Path where bounces are sent, often the ESP's own domain or a delegated subdomain of yours.
• bulk sender requirements - the rules Gmail and Yahoo enforce since February 2024 for senders of 5,000+ daily messages: authentication, a DMARC record, low complaint rates and working one-click unsubscribe. Taught in The 2024 bulk sender rules.
• CNAME - a DNS record that aliases one name to another. ESPs use CNAMEs for DKIM so they can rotate keys on their side without you editing DNS.
• cold email - unsolicited outreach to people with no prior relationship to you. Its deliverability economics are entirely different from permission-based mail.
• complaint rate - the share of delivered messages recipients mark as spam. Gmail's enforced ceiling is 0.3%, with under 0.1% the stated target.
• DBL - Spamhaus's Domain Block List: domain reputation rather than IP reputation. A DBL listing follows the domain onto any sending infrastructure.
• dedicated IP - a sending IP used by one sender alone, whose reputation is entirely yours to build and to ruin. Compared in Shared vs dedicated IPs.
• deliverability - whether the mail you send reaches inboxes. Distinct from delivery: mail can be accepted and still buried in spam.
• deliverability decay - the observation that working email setups break silently over time through DNS edits, rule changes, expiring assumptions and list aging. The second section of this course, starting at What is deliverability decay, is about it.
• delisting - removal from a blocklist, requested through the operator's process after the cause is fixed. Free at every reputable list.
• delivery rate - the share of sent messages accepted by receiving servers. What ESP dashboards report; its limits are taught in Measuring deliverability.
• disposition - in a DMARC report, what the receiver actually did with failing mail: none, quarantine or reject.
• DKIM - DomainKeys Identified Mail (RFC 6376): a cryptographic signature proving the signing domain handled the message and the signed parts were not altered. Taught in DKIM explained.
• DMARC - the policy layer (RFC 7489) that ties SPF and DKIM to the visible From domain and tells receivers what to do with failures. Taught in DMARC explained.
• DNS - the Domain Name System, the public database where every email authentication record lives. Taught in DNS for email.
• DNSBL - a blocklist queried over DNS, the mechanism behind most IP and domain blocklists.
• domain reputation - a mailbox provider's accumulated judgment of a sending domain, built from how recipients and filters have treated its mail over time. Taught in Sender reputation.
• double opt-in - confirming each subscription with a click in a confirmation email, so bots and typos never enter your list.

E-H

• engagement - the signals recipients generate about your mail: opens, clicks, replies, deletions and spam reports. Filters weigh them per recipient and in aggregate.
• envelope - the SMTP-level addressing of a message (MAIL FROM and RCPT TO), separate from the headers readers see. Taught in Anatomy of an email.
• envelope sender - the MAIL FROM address in the envelope, recorded at delivery as the Return-Path. SPF evaluates its domain.
• ESP - email service provider: the platform that sends your marketing or transactional mail (Mailchimp, Kit, Brevo). The cast of characters is mapped in Who is who in email.
• FBL - feedback loop: a mailbox provider's program that reports spam complaints back to the sender so complainers can be removed. Taught in Bounces, complaints and feedback loops.
• FCrDNS - forward-confirmed reverse DNS: the sending IP resolves to a hostname that resolves back to the same IP. Required of bulk senders; mail without it looks like mail from nowhere.
• forwarding - one mailbox passing a message on to another. It changes the sending IP, which breaks SPF, and it appears in DMARC reports as small unexplained failures.
• From - the sender address readers see (RFC5322.From). DMARC alignment is measured against its domain, precisely because it is the address humans trust.
• Google Postmaster Tools - Google's free dashboard for senders: domain reputation and spam-rate charts for your Gmail traffic. Taught in Measuring deliverability.
• graymail - legitimately subscribed mail that recipients no longer read. A slow poison for engagement metrics.
• greylisting - a receiver temporarily rejecting mail from an unknown source and accepting the retry. Real mail servers retry; much spamware does not.
• hard bounce - a permanent delivery failure, typically an address that does not exist. Repeatedly mailing hard-bouncing addresses is a strong spam signal.
• header - one name-value line in a message's metadata block: From, Subject, Received and dozens more.

I-L

• inbox placement - which folder delivered mail actually lands in. No sender-side tool measures it directly; see Measuring deliverability.
• include - the SPF mechanism (include:) that pulls another domain's authorized senders into your record. Each one spends DNS lookups against the limit of 10.
• IP reputation - the reputation attached to a sending IP address, as distinct from its domain. On shared infrastructure it is collective property.
• JMRP - Microsoft's Junk Mail Reporting Program, its feedback loop for Outlook.com complaints.
• list hygiene - the discipline of removing dead and disengaged addresses before they hurt you. Taught in List hygiene.
• List-Unsubscribe - the header (RFC 2369) that advertises how to unsubscribe without hunting through the message body.
• lookup limit - SPF's hard cap of 10 DNS lookups per evaluation. Exceeding it is a permerror, and accumulating includes is how mature setups drift across it.

M-P

• mailbox provider - the service operating recipients' inboxes and running the filters that decide placement: Gmail, Outlook, Yahoo, corporate systems.
• MAIL FROM - the SMTP command naming the envelope sender. Its domain is what SPF checks.
• Message-ID - a unique identifier header every legitimate message should carry. Malformed or missing ones correlate with spam.
• MIME - the standard (RFC 2045) that lets email carry HTML, images and attachments rather than plain text only.
• mitigation - a mailbox provider manually easing a block or filtering decision for a sender, usually temporary and conditional on behavior. See Deliverability triage.
• MTA - mail transfer agent: the server software that moves mail between systems.
• MTA-STS - a policy (RFC 8461) letting a domain require encrypted delivery for mail it receives. Covered in Beyond the big three.
• MX record - the DNS record naming the servers that receive a domain's mail. Domains that cannot receive mail look disposable to filters.
• one-click unsubscribe - the RFC 8058 version of List-Unsubscribe that works with a single automated request. Gmail and Yahoo require bulk senders to offer it and honor it within two days.
• open rate - the share of delivered messages recorded as opened, via tracking pixel. Apple's prefetching has inflated it since 2021, so only its trend carries signal.
• PBL - Spamhaus's Policy Block List: IP ranges that should not be sending mail directly, such as home connections. A PBL listing describes address-space policy rather than sender behavior.
• pct - the DMARC tag that applies your policy to only a percentage of failing mail. A forgotten pct=10 quietly weakens a strict-looking policy.
• permerror - a permanent SPF evaluation error, caused for example by two records on one domain or by exceeding the lookup limit. Receivers get no usable SPF result from then on, which under DMARC counts as not a pass.
• phishing - mail impersonating a trusted sender to steal credentials or money. Enforced DMARC exists chiefly to make impersonating your domain fail.
• policy (p=) - the DMARC tag telling receivers what to do with failing mail: none observes, quarantine spam-folders, reject refuses.
• postmaster - the traditional address and role for whoever runs a domain's mail. Providers publish their sender rules on postmaster sites.
• PTR record - the DNS record mapping an IP address back to a hostname; the reverse half of FCrDNS.

Q-T

• quarantine - the DMARC policy value asking receivers to treat failing mail with suspicion, which in practice means the spam folder.
• Received header - the line each handling server stamps onto a message. The stack of them, read bottom-up, is the delivery route. See Reading email headers.
• report processor - a service that ingests DMARC aggregate reports and turns IP-keyed rows into named sources and trends. See Reading DMARC reports.
• Return-Path - the header recording the envelope sender at delivery. Bounces go there, and SPF evaluates its domain.
• reverse DNS - resolving an IP address back to a hostname. Sending IPs without it fail a basic legitimacy test.
• RFC - Request for Comments: the numbered documents where internet standards live. SMTP is RFC 5321, SPF 7208, DKIM 6376, DMARC 7489.
• rua - the DMARC tag naming the address where aggregate reports go. Publishing one turns on your radar; see Reading DMARC reports.
• ruf - the DMARC tag requesting per-message forensic reports. Mostly unanswered today, for privacy reasons.
• SBL - the Spamhaus Block List: IPs observed sending spam. The classic behavior-based listing.
• seed testing - estimating placement by mailing a panel of test addresses at many providers. Limited, because seeds have no engagement history with the sender.
• selector - the name that chooses which DKIM key signs a message, published in DNS at <selector>._domainkey.<domain>. Taught in DKIM explained.
• sender reputation - the umbrella term for the domain and IP reputations a sender accumulates. Taught in Sender reputation.
• shared IP - a sending IP used by many of an ESP's customers at once, pooling their reputations for better and for worse.
• SMTP - Simple Mail Transfer Protocol (RFC 5321), the protocol servers use to hand mail to each other. Taught in What happens when you hit send.
• SNDS - Smart Network Data Services, Microsoft's IP-level dashboard showing how your traffic to Outlook mailboxes was treated. Requires control of the sending IPs. Taught in Measuring deliverability.
• soft bounce - a temporary delivery failure, such as a full mailbox. Retried automatically, and only a problem in bulk.
• spam trap - an address that exists only to catch senders with bad list practices: mail to it proves the list was not built on real consent or is not being cleaned. Covered in List hygiene.
• SPF - Sender Policy Framework (RFC 7208): the DNS record listing which servers may send mail using your domain's envelope address. Taught in SPF explained.
• spoofing - sending mail with a forged From address. What the whole authentication stack exists to defeat.
• subdomain - a name under your domain, like news.example.com, usable to separate mail streams and their reputations. See Separate your mail streams.
• sunset policy - a standing rule that stops mail to addresses after a defined period without engagement.
• suppression list - the do-not-mail list your ESP maintains: everyone who unsubscribed, complained, bounced hard, or was manually excluded.
• temperror - a temporary authentication evaluation error, usually a DNS timeout. Transient, unlike permerror.
• TLS - the encryption protecting mail in transit between servers. Its absence on a sending path is a modern warning sign.
• TLS-RPT - reporting (RFC 8460) about failures to deliver mail to your domain over TLS. The companion to MTA-STS.
• tracking pixel - the invisible one-pixel image whose load is counted as an open. The mechanism Apple's prefetching defeats.
• transactional email - mail triggered by a user's action, like receipts and password resets, as opposed to campaigns. Keep it on its own stream: Separate your mail streams.
• triage - working a deliverability incident by checking causes in order of measurability and likelihood instead of guessing. The runbook is Deliverability triage.
• TTL - time to live: how long DNS answers may be cached. The reason record changes take time to propagate, and the reason a deleted record keeps "working" for a while.

U-Z

• warming - gradually increasing volume on a new IP or domain so filters can build a history. Real, but oversold: warming introduces new capacity politely, it does not repair a reputation problem.
• XBL - the Spamhaus Exploits Block List: compromised machines sending spam without their owners' knowledge, such as infected computers and hijacked servers.
• ZEN - Spamhaus's combined IP zone, the list most receivers actually query. One lookup covers all three component lists.
• zone - the portion of DNS a nameserver is authoritative for. Your email records live in your domain's zone, wherever its DNS is hosted.