Email Marketing & Validation Glossary
A reference guide to email validation, address verification, deliverability, and sender authentication terms: SMTP checks, MX records, SPF / DKIM / DMARC, hard bounce and soft bounce, spam traps, catch-all, disposable email, domain warm-up, and sender score. Clear, no-fluff definitions. 106 terms.
A record and email
An A record maps a domain name to an IPv4 address. Learn how A records support MX hosts, act as a delivery fallback, and interact with SPF and PTR records.
Accept-all domain
An accept-all domain accepts SMTP mail to any address, real or not. Learn why this makes email validation inconclusive and how to manage it in your lists.
ARC Authentication
ARC preserves SPF, DKIM, and DMARC results across email forwarding hops. Defined in RFC 8617 and supported by Gmail, Microsoft 365, and Yahoo.
BIMI record
What a BIMI record is, how it works with DMARC enforcement, SVG logo requirements, VMC certificates, and which mail providers support it.
Bot Signups
Bot signups flood your email list with fake, stolen, or random addresses. Learn how to spot them and stop them before they wreck your sender reputation.
Bulk Email Verification
What bulk email verification is, how batch processing works, output statuses, and when to run a list clean.
CAN-SPAM Act
CAN-SPAM Act is the US federal law governing commercial email. Covers the seven requirements, opt-out rules, penalties up to $51,744 per message, and what it means for email deliverability.
Catch-all email
A catch-all (accept-all) domain accepts mail to any address, real or invented. Here's why that breaks SMTP validation and what to do about it.
CNAME record and email
A CNAME record creates a domain alias. For email infrastructure it has hard limits: it cannot coexist with MX, breaks SPF, and affects DKIM and DMARC configurations.
Complaint Addresses
Complaint addresses are subscribers who hit Spam. One complaint does more damage than a thousand unsubscribes — here is why and what to do about it.
Complaint rate
Complaint rate: the share of delivered emails marked as spam. Learn the 0.1% Google threshold, FBL mechanics, and how to keep your rate low.
Confirmed opt-in
Confirmed opt-in (COI) adds a verification click before activating a subscriber. Learn how it differs from double opt-in, when it is legally required, and why list quality improves.
Corporate Email
Corporate email is an address on your own domain. How it differs from free providers, why it affects deliverability, and what to watch for in B2B lists.
Dedicated IP for email
A dedicated IP gives you full control over your sender reputation. Here's when it's worth the trouble and what setup actually involves.
Disposable Email Address (DEA)
Disposable email addresses (DEA, temp mail) live 10 minutes to a day. Learn how to detect them and why filtering them protects deliverability.
DKIM Signature
DKIM (DomainKeys Identified Mail) is a cryptographic signature added to every outbound email. Learn how it works, how to configure it, and why key rotation matters.
DMARC alignment
DMARC alignment ties the visible From domain to SPF and DKIM results. Without it, both protocols can pass and DMARC still fails.
DMARC policy
DMARC is a DNS record that tells mail servers what to do when SPF and DKIM fail. Covers alignment, the three policy levels, record tags, reports, and common mistakes.
DNS email validation
How DNS validation works in email verification: MX, A, and TXT record checks, what each tells you, and where DNS fits in the validation pipeline.
DNS propagation
DNS propagation is how updated DNS records spread across recursive resolvers worldwide. TTL controls the speed, not the "24-48 hours" rule of thumb.
DNS TXT record
A DNS TXT record stores arbitrary text used for SPF, DKIM, DMARC, domain ownership proofs, and more. Format, length limits, common mistakes, and email authentication explained.
DNSBL (DNS-Based Blackhole List)
What DNSBL is, how DNS-based blackhole list queries work, the major lists (Spamhaus, Barracuda, SpamCop), why IPs get listed, and how to get delisted.
Domain Age Check
Domain age check: how a domain's registration date signals email risk. New domains under 7 days are high-risk; learn the thresholds, WHOIS/RDAP lookup, and scoring logic.
Domain Reputation
Domain reputation is the trust score mailbox providers assign to a sending domain. Learn what shapes it, how it differs from IP reputation, and how to check it.
Domain Warming
Domain warming: gradually ramping up email volume from a new domain to build sender reputation with mailbox providers before bulk sending.
Double opt-in
Double opt-in requires a confirmation click before adding a subscriber. Learn how it works, why it cuts bounces, and when it is legally required.
DSN (Delivery Status Notification)
What a Delivery Status Notification is, how RFC 3464 structures the report, what status codes mean, and how ESPs parse bounces automatically.
Email append
Email append matches your CRM records against a third-party database to fill in missing email addresses. Here's how it works, the legal risks, and the data quality problems.
Email authentication records
SPF, DKIM, DMARC, and BIMI are DNS records that prove a sender's identity. Required by Gmail and Yahoo for bulk senders since February 2024.
Email Blacklist Check
What email blacklist checks are, how DNSBL lookups work, the major lists, why IPs get listed, and how to get removed.
Email Bounce Rate
Email bounce rate: the percentage of undelivered messages. Covers hard bounce vs. soft bounce, acceptable thresholds, causes, and how to reduce bounces.
Email click-through rate (CTR)
Email CTR is the percentage of delivered messages that earned at least one click. Formula, difference from CTOR, industry benchmarks, and what actually moves it.
Email Content Filtering
Email content filtering is how spam filters analyze message body, HTML, links, and headers to assign a spam score and decide where your mail lands.
Email Deduplication
Email deduplication removes duplicate addresses from a mailing list. Learn why duplicates appear, how to find them, and which merge strategy to apply.
Email deliverability
Email deliverability is how reliably your messages reach the inbox. Learn how sender reputation, SPF/DKIM/DMARC, list quality, and content filtering each affect it.
Email Gateway
An email gateway is a server that filters and routes all inbound and outbound mail. Learn about SEG, DLP, spam filtering, encryption, and how gateways affect deliverability.
Email Headers
Email headers are the metadata fields that control routing, authentication, and display. From, Received, DKIM-Signature, Authentication-Results, and how to read them.
Email List Hygiene
Email list hygiene means regularly removing invalid, inactive, and risky addresses. Learn which contacts to cut, how often, and why it protects your deliverability.
Email List Segmentation
Email list segmentation splits subscribers into groups by behavior, demographics, or purchase stage to send more relevant campaigns and improve deliverability.
Email Open Rate
Email open rate: the share of delivered messages recipients opened. How tracking pixels work, Apple MPP impact, industry benchmarks, and what actually moves opens.
Email risk score
Email scoring assigns a numeric risk score to each address based on SMTP status, domain type, MX config, and risk signals. Learn what the score measures and how to act on it.
Email Spoofing
Email spoofing is forging the sender address in an email. Learn how attackers exploit SMTP, the difference between SPF, DKIM, and DMARC, and how to protect your domain.
Email Suppression List
A suppression list is the set of addresses you must never mail: unsubscribes, spam complainers, hard bounces, and spam traps. Here's how it works and why losing it hurts.
Email Syntax Validation
Checking an email address format against RFC 5321/5322 rules: allowed characters, structure, and length.
Email Throttling
Email throttling limits how fast a sender can deliver messages to a mail server. Why providers throttle, Gmail and Outlook limits, and how to configure your own rate.
Email Typo Detection
How email validators catch domain typos like gmial.com and suggest corrections — algorithms, thresholds, and where to apply it.
Email Validation API
An email validation API checks addresses in real time via HTTP: syntax, DNS, SMTP, disposable detection. How it works and when to use it.
Email Validation Webhook
An email validation webhook is a POST request your validation service sends to your server when a check completes. Push delivery, payload structure, security, retry logic.
Email verification accuracy
Accuracy, precision, recall, FPR, FNR: the metrics that reveal whether an email validator is trustworthy — and how to compare services honestly.
Email Warmup Service
Email warmup services automatically build sender reputation for new domains and IPs. Learn how they work, what the real risks are, and when manual warmup is the smarter choice.
Envelope From and Header From
Every email has two sender addresses. Envelope From handles bounce routing; Header From is what recipients see. Here is how they differ and why DMARC cares.
ESP — Email Service Provider
What an ESP is and how it works: SMTP relay, IP pool management, bounce handling, authentication. How to choose between shared and dedicated infrastructure.
Feedback loop (FBL)
An email feedback loop lets ISPs report spam complaints back to senders. Learn how FBL and ARF work, which providers offer them, and why your complaint rate matters.
Free Email Provider
Free email providers give users a shared-domain address at no cost. Learn how Gmail, Outlook, and Yahoo behave during SMTP verification and why provider type matters for list quality.
GDPR and Email Marketing
GDPR rules for email marketing: consent requirements, subscriber rights, data processing obligations, and penalty tiers for EU violations.
Google Postmaster Tools
Google Postmaster Tools is Google's free dashboard at postmaster.google.com that shows how Gmail rates your sending domain and IP. Learn what it reports and how to act on it.
Greylisting
Greylisting is an anti-spam technique that temporarily rejects email from unknown senders and waits to see if the sending server retries.
Hard Bounce
A hard bounce is a permanent email delivery failure. Learn the SMTP codes, causes, deliverability risks, and how to prevent them.
Honeypot form fields
A honeypot field is a hidden form input that only bots fill in. Learn how it works, how to implement it correctly, and why it beats CAPTCHA for signup forms.
IMAP Protocol
IMAP keeps email on the server and syncs state across devices. Ports 143/993, folder flags, IDLE push, CONDSTORE resync, and when to use it over POP3.
Inactive email subscribers
Inactive subscribers stop opening and clicking your emails. Learn when to flag them, why they drag down deliverability, and how to run a re-engagement sequence.
Inbox placement rate
Inbox placement rate (IPR) is the share of sent emails that land in the inbox, not spam. Learn how to measure it, what benchmarks to target, and how to improve it.
IP Reputation
IP reputation is the trust score mailbox providers assign to your sending IP. Learn the difference between dedicated and shared IPs, how to check your score, and how to recover.
IP warming
IP warming is the practice of gradually increasing email volume from a new IP to build sender reputation. Learn the schedule, metrics, and mistakes to avoid.
List-Unsubscribe Header
The List-Unsubscribe header lets email clients show a one-click unsubscribe button. Required for bulk senders since 2024 under Gmail and Yahoo rules.
Mail Queue
A mail queue is the internal buffer where an MTA holds messages pending delivery. Learn how it works, why it grows, and how to monitor and manage it.
Mail server
A mail server is software that receives, stores, and forwards email. Covers MTA, MDA, MUA, DNS records, and how SMTP validation interacts with the server.
Mail Transfer Agent (MTA)
An MTA is server software that routes and relays email between mail servers over SMTP. Covers Postfix, Exim, mail queues, authentication, and deliverability.
Mailbox Existence Check
A mailbox existence check uses SMTP to confirm whether a specific address accepts mail. How RCPT TO works, why catch-all domains complicate results, and accuracy limits.
MDA - Mail Delivery Agent
An MDA receives mail from the MTA and places it into the recipient's mailbox. How Dovecot, Procmail, Maildir, and mbox fit into email delivery.
Message-ID
Message-ID is the RFC 5322 header that gives each email a globally unique identity. Learn how it works for deduplication, threading, and delivery debugging.
MIME Format
MIME extends email beyond plain ASCII: attachments, HTML, images, non-Latin text. Covers multipart structure, boundary, base64, quoted-printable, and Content-Type.
MUA (Mail User Agent)
A Mail User Agent is the software you use to compose, send, and read email. Learn how Thunderbird, Outlook, and webmail clients fit into the email delivery chain.
MX Record
MX (Mail Exchange) record: how DNS routes email to the right server, priority values, null MX (RFC 7505), common misconfigurations, and its role in email validation.
MX Record Check
How checking a domain's MX record works during email validation and what it tells you about deliverability.
Open Relay
An open relay is a mail server that forwards email for anyone without authentication. Learn why it's a critical misconfiguration and how to fix it.
Poisoned Email Data
Poisoned email data: fake, malicious, and planted addresses that corrupt a mailing list. How they get in, why they matter, and how to remove them.
POP3 protocol
POP3 (RFC 1939) downloads email from a server to a local device, then deletes it. How the three session phases work, which ports to use, and when POP3 still makes sense.
Postmaster
What a postmaster is, why postmaster@ is required by RFC 5321, and how Google Postmaster Tools and similar dashboards work.
PTR Record
A PTR record maps an IP address back to a hostname via reverse DNS. Learn why mail servers need one and how FCrDNS works.
Real-time email verification
Real-time email verification checks an address the moment it is entered — via SMTP, MX, and DNS — before it reaches your database. Here's how it works.
Received Header
The Received header is the trace each mail server stamps on a message in transit. Learn to read the routing chain, diagnose delays, and spot forged hops.
Return-Path
Return-Path is the email header that tells receiving servers where to send bounces. It comes from MAIL FROM in SMTP, affects SPF checks, and is central to VERP and DMARC alignment.
Role-Based Email Address
Role-based addresses (info@, support@, admin@) belong to a function, not a person. Why they hurt deliverability and how to handle them.
Russian Advertising Law (38-FZ) and Email Marketing
Federal Law 38-FZ governs commercial email in Russia. Article 18 requires prior consent, places the proof burden on senders, and carries fines up to 500,000 RUB.
Seed list testing
Seed list testing measures inbox placement by sending to control addresses across Gmail, Outlook, Yahoo, and more. See how it works and when to use it.
Sender Reputation
Sender reputation is the score mailbox providers assign to your sending source. Learn what drives it, how to check it, and how to recover when it drops.
Sender Score
Sender Score is a 0-100 IP reputation rating from Validity. Learn what drives it, how to read the tiers, and why it differs from Gmail's own reputation system.
Single opt-in
Single opt-in adds subscribers instantly on form submit, no confirmation required. Learn the tradeoffs for list quality, deliverability, and compliance.
SMTP Protocol
SMTP (Simple Mail Transfer Protocol) is how email moves between servers. Learn commands, ports 25/465/587, response codes, and how SMTP validates addresses.
SMTP Relay Server
An SMTP relay server is an intermediary MTA that forwards email toward its destination. Learn smart host config, open relay risks, cloud services, and deliverability.
SMTP relay service
An SMTP relay service is cloud infrastructure that accepts email from your app and delivers it. SendGrid, Amazon SES, Mailgun, Postmark: architecture, setup, and trade-offs.
SMTP Response Codes
Three-digit SMTP response codes explained: 2xx, 3xx, 4xx, 5xx, enhanced RFC 3463 codes, and how they affect bounce handling and email validation.
SMTP Verification
SMTP verification checks whether an email address exists by connecting to the mail server without sending a message. How it works, response codes, and limits.
Soft bounce
A soft bounce is a temporary email delivery failure. Learn what causes SMTP 4xx codes, how retry logic works, and when to remove soft-bouncing addresses.
Spam Filter
How spam filters work: connection checks, SPF/DKIM/DMARC authentication, content scoring, and engagement signals. What causes inbox failures and how to fix them.
Spam Folder
The spam folder is where mailbox providers route messages they classify as unwanted. How placement decisions work, why legitimate mail ends up there, and how to stay out.
Spam Score
What spam score is, how filters like SpamAssassin calculate it, what raises or lowers it, and how to test your messages before sending.
Spam Traps
What spam traps are, the three types (pristine, recycled, typo), how they reach your list, and how to keep them out.
Spamhaus
Spamhaus runs the SBL, XBL, PBL, and DBL blocklists used by Gmail, Outlook, and most major mail providers. Here's how they work and how to get off them.
SPF Record
SPF is a DNS TXT record that authorizes mail servers to send on your domain's behalf. Learn syntax, the 10-lookup limit, common mistakes, and how SPF fits with DKIM and DMARC.
STARTTLS
STARTTLS upgrades a plaintext SMTP, IMAP, or POP3 connection to TLS in-place. Learn how the handshake works, downgrade attacks, MTA-STS, DANE, and Postfix config.
TLS email encryption
TLS secures email in transit between servers and clients. Covers TLS 1.2 vs 1.3, STARTTLS vs implicit TLS, opportunistic vs enforced, MTA-STS, DANE, and deliverability impact.
Transactional email
Transactional emails are automated messages triggered by user actions. Learn how they differ from marketing email and why they need separate sending infrastructure.
Unknown Email Status
Unknown email status means the validator couldn't confirm whether a mailbox exists. Learn the causes: SMTP timeouts, greylisting, rate limiting, and how to handle these addresses.
Unsubscribe rate
Unsubscribe rate: the share of delivered emails that prompt an opt-out. Formula, industry benchmarks, causes, and how to bring the number down.