Email deliverability glossary
The terms that decide whether your cold email reaches the inbox, defined in plain English with the context that matters for senders.
Authentication
- Authenticated Received Chain (ARC)A protocol that preserves email authentication results across forwarding and intermediaries.
- Brand Indicators for Message Identification (BIMI)A standard that displays your verified brand logo next to authenticated messages in supporting inboxes.
- DKIM SelectorThe label in a DKIM signature that tells receivers which public key to fetch from DNS to verify the message.
- DMARC AlignmentThe requirement that an authenticated SPF or DKIM domain match the visible From domain for DMARC to pass.
- DMARC PolicyThe instruction in a DMARC record (none, quarantine, or reject) telling receivers how to handle mail that fails authentication.
- Domain-based Message Authentication, Reporting and Conformance (DMARC)A policy layer built on SPF and DKIM that tells receivers how to handle mail that fails authentication and where to send reports.
- DomainKeys Identified Mail (DKIM)An authentication method that cryptographically signs outgoing email so recipients can verify it was not altered and came from the claimed domain.
- Email Authentication (SPF, DKIM, DMARC)The DNS-based protocols that prove an email genuinely came from your domain.
- Header From (From: Header)The visible From address recipients see, and the identity DMARC alignment is measured against.
- Sender Policy Framework (SPF)An email authentication standard that lets a domain publish which mail servers are authorized to send on its behalf.
- SPF FlatteningReplacing the include mechanisms in an SPF record with resolved IP addresses to stay under the DNS lookup limit.
- SPF PermError (Permanent Error)A permanent SPF evaluation failure caused by an invalid or unprocessable SPF record, most often from exceeding the 10 DNS-lookup limit.
Compliance
- CAN-SPAM ActThe US federal law setting rules for commercial email, including honest headers, a valid physical address, and a working opt-out.
- CASL (Canada's Anti-Spam Legislation)Canada's strict consent-based email law requiring permission before sending commercial messages to Canadian recipients.
- Cold EmailUnsolicited outreach sent to a recipient who has no prior relationship with the sender, typically for B2B sales prospecting.
- GDPR (General Data Protection Regulation)The EU data-protection law governing how personal data, including business email addresses, is collected and used for outreach.
- List-Unsubscribe HeaderAn email header (RFC 2369) that gives mailbox providers a machine-readable way to let recipients unsubscribe directly from the inbox.
- One-Click Unsubscribe (RFC 8058)A standard that lets a recipient unsubscribe with a single click, with no confirmation page or login, via an automated POST request.
Content
- Apache SpamAssassinA widely used open-source spam filter that scores email against hundreds of rules to decide whether it is spam.
- Drip CampaignAn automated series of emails delivered on a fixed schedule or in response to recipient behavior.
- Email SequenceAn ordered set of automated emails, most often an initial message plus timed follow-ups, sent until a recipient responds or the sequence completes.
- Marketing EmailA bulk commercial message promoting products, offers, or content sent to a list of opted-in recipients.
- Merge TagA placeholder token in an email template that is replaced with recipient-specific data from a list or CRM at send time.
- PersonalizationTailoring an email's content to the individual recipient using their attributes or context rather than sending an identical message to everyone.
- Spintax (Spinning Syntax)A syntax using brace-delimited alternatives like {hi|hello|hey} to generate many slightly different versions of the same message.
- Transactional EmailA one-to-one email triggered by a specific user action or account relationship, such as a password reset or receipt.
DNS
- A Record (Address Record)A DNS record that maps a hostname to an IPv4 address.
- CNAME Record (Canonical Name Record)A DNS record that aliases one hostname to another, commonly used for custom tracking domains and delegated DKIM.
- DNS PropagationThe delay before a DNS change is visible everywhere as cached copies of the old record expire.
- Domain Name System (DNS)The global directory that maps domain names to IP addresses and stores the records that govern email routing and authentication.
- Mail Transfer Agent Strict Transport Security (MTA-STS)A policy that forces inbound mail to your domain to be delivered over validated, encrypted TLS.
- MX Record (Mail Exchange Record)A DNS record that specifies which mail servers receive email for a domain, in priority order.
- NS Record (Name Server Record)A DNS record that delegates a domain to the authoritative name servers responsible for its zone.
- PTR Record (Pointer Record)A reverse-DNS record that maps a sending IP address back to a hostname.
- Reverse DNS (rDNS)The lookup that resolves an IP address back to a hostname, the opposite of normal DNS resolution.
- SubdomainA prefixed segment of your root domain (e.g. mail.example.com) used to isolate sending reputation.
- TTL (Time To Live)The number of seconds DNS resolvers are allowed to cache a record before re-querying it.
- TXT RecordA DNS record type that holds arbitrary text, used to publish SPF, DKIM, and DMARC authentication policies.
Deliverability
- Blacklist (DNSBL / Blocklist)A published list of IP addresses or domains known for spam that mail servers consult to block or filter incoming messages.
- Bulk Sender Requirements (Gmail and Yahoo 2024)The 2024 Gmail and Yahoo rules high-volume senders must meet to keep reaching the inbox: authentication, easy unsubscribe, and a low spam rate.
- DeliverabilityThe ability of your emails to reach recipients' inboxes rather than being blocked or filtered to spam.
- Domain ReputationA mailbox provider's trust score for your sending domain, built from authentication, engagement, complaints, and spam-trap hits.
- Feedback Loop (FBL)A program by which a mailbox provider forwards spam complaints back to the sender so they can suppress those recipients.
- GreylistingAn anti-spam tactic where a server temporarily rejects mail from unknown senders and accepts it only on a retry.
- Hard BounceA permanent delivery failure, usually because the recipient address does not exist.
- Inbox PlacementThe share of delivered emails that land in the primary inbox rather than spam or a secondary tab.
- IP ReputationA mailbox provider's trust score for the specific IP address that delivers your mail.
- Microsoft SNDS (Smart Network Data Services)Microsoft's free portal that reports IP reputation, traffic, and spam data for senders mailing into Outlook, Hotmail, and Live.
- Promotions TabGmail's tabbed-inbox category where bulk, promotional, and marketing-style mail is automatically sorted, separate from the Primary tab.
- Realtime Blackhole List (RBL)A DNS-queryable blacklist that lets receiving servers check a sender's IP reputation in real time during the SMTP handshake.
- Seed ListA set of monitored test inboxes across major providers used to measure where your email actually lands.
- Sender ReputationA mailbox provider's trust score for a sending IP and domain that drives inbox placement.
- Sender ScoreA 0 to 100 reputation rating that estimates how trustworthy a sending IP address looks to mailbox providers.
- Snowshoe SpamA spam technique that spreads sending across many IP addresses and domains to keep any single one below reputation-based detection thresholds.
- Soft BounceA temporary delivery failure that may succeed on a later retry.
- Spam ComplaintThe event of a recipient marking a message as spam or junk, signaling the mailbox provider that the mail was unwanted.
- Spam FolderThe junk folder where mailbox providers quarantine messages judged to be unwanted or risky.
Infrastructure
- Backscatter (Collateral Spam)Misdirected bounce messages or auto-replies sent to an innocent address that was forged as the sender of spam.
- Custom Tracking DomainA branded domain or subdomain you own, pointed at your ESP's tracking host, used to serve open pixels and click-redirect links under your own name.
- Dedicated IP AddressA sending IP address used by only one sender, giving you sole ownership of its reputation.
- Domain WarmingGradually ramping volume and engagement on a new sending domain to establish reputation.
- Envelope Sender (MAIL FROM)The address given in the SMTP MAIL FROM command that identifies the true sender for routing and bounces.
- ESP (Email Service Provider)A platform that hosts mailboxes or sends email on your behalf, managing delivery infrastructure and authentication.
- IMAP (Internet Message Access Protocol)A protocol for reading and managing email on a remote mail server while messages stay stored server-side.
- IP WarmingGradually increasing send volume on a new IP to build a positive sending history.
- Mail Transfer Agent (MTA)The server software that routes and delivers email between domains using SMTP.
- Mailbox Provider (MBP)The company that operates the receiving email infrastructure and inboxes for recipients, such as Google, Microsoft, or Yahoo.
- POP3 (Post Office Protocol version 3)An older protocol that downloads email to a single client and usually removes it from the server.
- Return-PathThe address where bounces and delivery failures are sent, and the domain SPF authenticates against.
- Shared IP AddressA sending IP used by multiple senders who collectively shape its reputation.
- Simple Mail Transfer Protocol (SMTP)The internet standard protocol for sending and relaying email between servers.
- SMTP (Simple Mail Transfer Protocol) Error CodeThe numeric reply code an SMTP server returns to indicate whether a message was accepted, temporarily deferred, or permanently rejected.
- SMTP RelayA mail server or service that accepts outbound email and forwards it on toward the recipient's mail server.
- STARTTLSA command that upgrades a plaintext email connection to encrypted TLS on the same port.
- TLS (Transport Layer Security)The encryption protocol that secures email in transit between mail clients and servers.
- Tracking PixelA tiny transparent image embedded in an HTML email whose load from the server signals that the message was opened.
- URIBL (Uniform Resource Identifier Block List)A DNS-based blocklist of domains and URLs found inside message bodies, used to filter mail that links to known-bad sites.
List health
- Catch-All (Accept-All) DomainA domain that accepts mail for any address, real or not, making verification uncertain.
- Disposable Email (Disposable Email Address)A temporary, throwaway email address from a short-lived service that is designed to expire or be abandoned quickly.
- Double Opt-InA subscription method requiring the recipient to confirm via a follow-up email before being added to a list.
- HoneypotAn email address or web form field deliberately planted to catch spammers and scrapers, with no legitimate reason to ever receive mail.
- List HygieneThe ongoing practice of cleaning a contact list to remove invalid, risky, or unengaged addresses before sending.
- Role Account (Role-Based Address)A shared, function-based mailbox such as info@, sales@, or support@ that is tied to a role rather than a specific person.
- Single Opt-InA subscription method that adds a recipient to a list immediately after signup with no email confirmation step.
- Spam Trap (Honeypot)An email address used solely to catch senders with poor list hygiene or no permission.
Metrics
- Bounce RateThe percentage of sent messages that fail to deliver and bounce back.
- Click TrackingMeasuring link clicks by rewriting URLs to route through a redirect server that logs the click before forwarding the recipient.
- Complaint RateThe percentage of delivered emails that recipients mark as spam over a given period.
- Delivery RateThe percentage of sent emails accepted by recipient mail servers rather than bounced.
- Engagement RateA composite measure of how recipients interact with your emails, combining opens, clicks, and replies relative to messages delivered.
- Google Postmaster Tools (GPT)Google's free dashboard reporting how Gmail rates your domain's sending reputation and spam rate.
- Open RateThe percentage of delivered emails that recipients opened, usually measured via a tracking pixel.
- Open TrackingA technique that detects when a recipient opens an email, usually by embedding a tracking pixel that loads from a server.
- Reply RateThe percentage of delivered cold emails that generate a reply from the recipient.
- Spam Rate (Complaint Rate)The percentage of delivered emails that recipients mark as spam.
- Spam ScoreA numeric rating of how spammy a message looks, summed from rule hits by a content filter such as SpamAssassin.
- Unsubscribe RateThe percentage of delivered recipients who opt out of future messages via an unsubscribe link or header.
Know the terms. Now land in the inbox.
Understanding deliverability is step one. Scheduler Zero puts it into practice: domain warmup, authentication, and reputation management so your cold email actually arrives.