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. 47 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.
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.
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 (accept-all) domain
A domain that accepts mail for any address, even one that does not exist, and why SMTP verification cannot vouch for 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.
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.
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.
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 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 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 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.
Envelope From and Header From
Every email has two sender addresses. Envelope From (MAIL FROM, Return-Path) handles bounces; Header From is what recipients see. How they differ and why SPF and DMARC care.
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.
Greylisting
Greylisting is an anti-spam technique that temporarily rejects email from unknown senders and waits to see if the sending server retries.
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.
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.
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 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.
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.
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.
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.
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 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.
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.
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.
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.
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.