Email & Deliverability

Busted

• ARC (Authenticated Received Chain)A signed chain that carries earlier email authentication results through forwarders and mailing lists that may break SPF or DKIM.
• Authentication-Results headerThe receipt a mail system adds showing the SPF, DKIM, and DMARC results it calculated, including the domains each check used.
• Blocklist (RBL/DNSBL)Public lists of IPs and domains known for spam. Mail servers check them in real time and refuse or junk anything that shows up.
• Bounce processingTurning delivery failures into actions: retry temporary problems, suppress dead addresses, and investigate policy or reputation blocks.
• Bounce rateThe share of attempted recipient deliveries that receiving servers rejected, best split into permanent and temporary failures.
• Bulk email and marketing emailMail sent to a list because you decided to send it, not because one person triggered it. Newsletters and campaigns, held to stricter rules.
• Complaint feedback loop (FBL)A provider service that reports back when someone hits 'report spam' on your mail, so you can suppress them right away.
• Content-based spam filteringAn inbox filter that inspects an email's words, links, HTML, and layout for spam patterns, then combines those clues with other delivery signals.
• Custom MAIL FROM domain (branded Return-Path)Pointing the invisible bounce address at a subdomain you own, so SPF aligns with your brand instead of your email provider's domain.
• Dedicated vs shared IPShared IPs pool your mail with other senders; a dedicated IP isolates the history but makes its cost, warming, and reputation entirely yours.
• DKIMA signature on outgoing mail that ties the message to a signing domain and shows the covered headers and body were not altered on the way.
• DKIM key rotationReplacing a DKIM signing key by overlapping old and new selectors, so receivers can verify mail throughout the change.
• DKIM selectorThe name in a DKIM signature that tells the receiver which public key to fetch from the signing domain's DNS.
• DMARCA DNS policy telling inboxes what to do with mail using your domain in the From line when neither SPF nor DKIM passes in an aligned way.
• DMARC aggregate report (RUA)Periodic XML summaries from participating receivers, each listing the IPs it saw sending as your domain and how SPF, DKIM, and alignment went.
• DMARC identifier alignmentThe rule that SPF or DKIM must pass for the same domain shown in the From line, not just for some domain your provider owns.
• Domain warmingRamping a new sending domain from a trickle to full volume over weeks, so providers learn to trust you instead of flagging a sudden blast.
• Double opt-inNobody joins your list until they click a confirmation link you email them, so every subscriber proved the inbox is really theirs.
• Email address validationChecks an address for syntax, a mail-capable domain, and obvious risk before sending, without pretending it proves the mailbox belongs to someone.
• Email delivery event webhooksYour email provider POSTing to your app the moment a message is delivered, bounced, opened, or reported as spam, instead of you polling.
• Email service provider and SMTP relayOverlapping names for how mail gets out: campaign platforms, delivery providers, SMTP relays, and HTTP sending APIs, often from one vendor.
• Engagement signalsThe report card inboxes appear to keep on how recipients treat your mail, weighted by a formula the providers have never published.
• Envelope vs header FromEvery email has two From addresses: the invisible one bounces go to, and the one recipients see. They do not have to match.
• Gmail tab classificationGmail sorting inbox mail into categories such as Primary, Promotions, and Updates. Promotions is an inbox tab, not the spam folder.
• Hard bounceA failure the provider classifies as permanent. Retrying will not help, and address-level ones should be suppressed on the spot.
• Inbox placement vs delivery rateDelivery rate says the server accepted it. Inbox placement says whether it landed in Inbox or Spam. Your dashboard only shows the first.
• IP warmingGradually raising volume on a new dedicated sending IP so mailbox providers see a steady history before they receive your full load.
• Link tracking domainA branded host such as click.example.com that records an email click before redirecting, instead of putting a shared vendor domain in every link.
• List hygieneRoutinely pulling dead, bouncing, and long-silent addresses off your list so the people left are the ones who actually read you.
• List-Unsubscribe headerA header that tells mailbox apps which web or email address can unsubscribe the recipient, so they may show their own control.
• Message-ID / References headersHidden message identifiers that let inboxes connect a reply to its parent and reconstruct the rest of the email conversation.
• Multipart/alternative emailOne email carrying both plain-text and HTML versions of the same content, so each mail client can choose the format it can display.
• MX recordThe DNS record that tells other mail servers where to deliver email addressed to your domain, with lower preference numbers tried first.
• One-click unsubscribeTwo headers that let Gmail or Apple Mail unsubscribe someone in a single tap, with no landing page and no login required.
• Postmaster tools / domain reputation dashboardThe provider's own scoreboard for your domain: spam rate, reputation, and auth pass rates, straight from the inbox judging you.
• Reply-ToThe email header that sends a reader's reply to a different address while leaving the visible From address unchanged.
• Retry queueA holding area for temporarily failed messages, spacing out later attempts so an outage does not lose mail or trigger a retry storm.
• Reverse DNS (PTR)A DNS lookup from a sending IP back to its hostname, used by inbox providers to check whether the mail server has a credible identity.
• Seed list testingSending a campaign to controlled Gmail, Outlook, Yahoo, and other accounts to see the folder or tab it reaches before the real send.
• Sender reputationThe running score each mailbox provider keeps on your domain and IP, weighing complaints, bounces, and behavior by a formula only they see.
• SMTP deferral and greylistingA 'not now, try again' rejection. Real mail servers retry minutes later and get in; sloppy spam scripts never bother.
• SMTP reply codesThree-digit SMTP answers saying accepted, try again later, or permanently rejected, with extra status digits explaining why.
• Soft bounceYour provider's label for a failure it expects to clear: full mailbox, server hiccup, or you sending too fast. Back off, do not give up.
• Spam complaint rateHow often recipients hit the junk button, measured differently by each provider. Google wants its metric under 0.3 percent, ideally under 0.1.
• Spam trapA dead or planted address that exists only to catch senders mailing lists they never earned. Hitting one quietly wrecks your reputation.
• SPFA DNS record naming the IPs allowed to send with your domain in the SMTP envelope, checked by the receiver against the connecting server.
• Subdomain sendingSending each type of mail from its own subdomain, so a bad newsletter week is far less likely to drag your password resets down with it.
• Sunset policyA standing rule that stops mailing people who have ignored you for months, before their silence drags your reputation down.
• Suppression listYour do-not-send list: everyone who unsubscribed, hard bounced, or complained, with the reason and scope that says which mail is blocked.
• Throttling / rate limitingThe receiving server capping how much mail it will take from you at once, so a burst gets slowed down instead of delivered.
• Transactional emailMail your app sends to one person because they did something: receipts, password resets, invites. Expected, triggered, not a campaign.

The territory

30 core terms mapped for this field, ranked by how often builders reach for them. Each one is a future entry. Want to bust one? One entry, one file, one pull request.

• SPFDNS record listing servers allowed to send for your domain"the DNS thing that says who can send email as me" · "proving my server is allowed"
• DKIMCryptographic signature proving an email wasn't forged or altered"the signature key thing for email" · "signing my emails so they look legit"
• DMARCPolicy telling inboxes what to do when SPF/DKIM fail"the rule that stops people spoofing my domain" · "the p=reject thing"
• DMARC identifier alignmentVisible From domain matching authenticated SPF/DKIM domain"domain alignment" · "making the from address match the signed domain"
• Sender reputationScore mailbox providers assign your domain/IP based on behavior"my email credit score" · "whether Gmail trusts me"
• Domain warmingGradually ramping send volume to build sender reputation"starting slow so I don't get flagged" · "breaking in a new sending domain"
• Transactional emailOne-to-one triggered mail: receipts, resets, confirmations"the automatic emails the app sends" · "receipt and password reset mail"
• Bulk email and marketing emailBroadcast mail to lists, subject to stricter consent rules"newsletter blasts" · "campaign email"
• Hard bouncePermanent delivery failure; address is invalid or dead"the address doesn't exist" · "permanently undeliverable"
• Soft bounceTemporary failure: full mailbox, server down, throttled"bounced but might work later" · "temporary reject"
• Spam complaint ratePercent of recipients hitting "report spam"; keep under 0.3%"how many people mark me as junk" · "the complaint threshold"
• Complaint feedback loop (FBL)Provider-sent report when a recipient marks you spam"getting told who reported me" · "the complaint feed from Gmail/Yahoo"
• Suppression listAddresses you must never send to again"the do-not-send list" · "blocklist of my own unsubscribes"
• List hygieneRemoving dead, bouncing, and inactive addresses regularly"cleaning my list" · "pruning dead emails"
• Double opt-inSubscriber confirms via emailed link before receiving mail"make them click a confirm link" · "confirmed signup"
• One-click unsubscribeRequired header letting inboxes unsubscribe without a page"the unsubscribe button Gmail shows at the top" · "List-Unsubscribe-Post"
• Inbox placement vs delivery rateAccepted by server versus actually landing in Inbox"it says delivered but went to spam" · "where it actually lands"
• Blocklist (RBL/DNSBL)Public lists of IPs/domains inboxes refuse or filter"getting blacklisted" · "Spamhaus listing"
• Spam trapDead or planted address used to catch sloppy senders"the honeypot address that gets me flagged" · "poisoned email in my list"
• Envelope vs header FromBounce address versus the address recipients see"the From you see versus the real sender" · "MAIL FROM vs the visible From"
• Custom MAIL FROM domain (branded Return-Path)Your own bounce-handling domain, aligning SPF"custom return-path" · "the hidden bounce address"
• Subdomain sendingIsolating mail streams on subdomains to protect the root domain"send from mail.mysite.com" · "keeping marketing off my main domain"
• DMARC aggregate report (RUA)Daily XML report on who's sending as your domain"the DMARC report emails I keep getting" · "the daily XML from Gmail"
• Email delivery event webhooksProvider callbacks for delivered, bounced, opened, complained"webhook events" · "callbacks when my mail bounces"
• Engagement signalsOpens, replies, moves-to-inbox that providers use to rank you"whether people actually read it" · "the signals Gmail watches"
• Throttling / rate limitingProviders capping how fast you may deliver"sending too fast and getting deferred" · "4xx slow-down"
• SMTP deferral and greylistingTemporary rejection expecting a legitimate retry"rejected then accepted on retry" · "the wait-and-try-again reject"
• Postmaster tools / domain reputation dashboardProvider-side view of your spam rate and reputation"Google Postmaster Tools" · "the dashboard showing my spam rate"
• Sunset policyAutomatically stopping mail to long-inactive subscribers"cutting off people who never open" · "retiring dead subscribers"
• Email service provider and SMTP relayCampaign platform versus raw programmatic sending API"the thing that actually sends my mail" · "API vs newsletter tool"

Deeper in the field

• MX record DNS entry naming the server that receives your domain's mail
• Reverse DNS (PTR) Maps a sending IP to its mail server hostname
• DKIM selector Identifies the public key used to verify a DKIM signature
• DKIM key rotation Replaces signing keys periodically without interrupting authentication
• Authentication-Results header Shows receiver-evaluated SPF, DKIM, and DMARC outcomes
• ARC (Authenticated Received Chain) Preserves auth results across forwarding hops
• Reply-To Header routing replies somewhere other than the sender address
• Message-ID / References headers Threading identifiers grouping replies in inboxes
• List-Unsubscribe header Advertises supported unsubscribe methods to mailbox clients
• SMTP reply codes Distinguish acceptance, temporary deferral, and permanent rejection
• Retry queue Retains deferred messages and retries them with controlled backoff
• Bounce processing Classifies failures and suppresses permanently undeliverable recipients
• Bounce rate Percentage of attempted messages rejected by receiving servers
• Email address validation Screens malformed, disposable, or risky addresses before sending
• IP warming Gradually increases volume on a new dedicated sending IP
• Dedicated vs shared IP Sending from your own IP versus a pooled one
• Seed list testing Sending to test accounts across providers to check placement
• Gmail tab classification Gmail sorting mail into Primary, Promotions, Updates
• Link tracking domain Branded redirect domain for click tracking, avoids shared-domain taint
• Multipart/alternative email Text alternative bundled with HTML for filters and clients
• Content-based spam filtering Filters rating wording, links, and formatting risk