Every word email throws at you, defined.

The one communication channel you truly own: an asynchronous, protocol-driven system for reaching people directly, without a platform algorithm deciding who sees your message.

Email the recipient doesn't want, whether or not they technically consented. Usually bulk, sometimes deceptive, and defined by the reader's reaction as much as by permission.

Your track record as a sender, scored separately by each mailbox provider and at both domain and IP level. Authentication proves who you are; reputation proves you're worth delivering.

What recipients actually do with your mail: opens, clicks, replies, deletes, spam reports. Mailbox providers watch these signals to decide whether future sends reach the inbox.

The part of an email address after the @, and the identity mailbox providers build a lasting reputation around, even if you switch IPs or sending platforms.

Proof that an email really came from the domain in its From address. SPF, DKIM, and DMARC let receiving servers verify you instead of blocking or filtering you.

The numeric address of the mail server sending your message. Mailbox providers read it on connection and use its reputation history to make filtering decisions.

How mailbox providers decide where your email lands: inbox, spam, or blocked. Modern filters weigh engagement (does this user open you?) more heavily than message content.

Google's email service, launched 2004. Its scale and engagement-based filtering make it one of the mailbox providers whose sender requirements shape how everyone sends today.

The primary folder where a recipient sees mail they'll actually read. Landing here, not just being accepted by the server, is what deliverability really means.

Sender Policy Framework: a DNS record listing which servers may send mail for your domain. Receiving servers check it, but it breaks when mail is forwarded.

A policy layer on top of SPF and DKIM. It checks that the authenticated domain matches your visible From address, then tells receivers to monitor, quarantine, or reject failures.

Whether your email actually reaches the inbox, not just whether a server accepted it. It depends on authentication, list quality, and sender reputation working together.

The person you're sending to. Their individual behavior (opens, deletes, moving you in or out of spam) can override your global reputation for their own inbox.

DomainKeys Identified Mail: a cryptographic signature proving your message came from your domain and wasn't altered in transit. Receivers verify it using a public key in your DNS.

An email the receiving server refused, returned with a reason code. Hard bounces are permanent (the address is dead); soft bounces are temporary (full mailbox, server hiccup).

A mailbox provider: the company that hosts inboxes for end users, then receives, filters (inbox vs spam), and stores their mail. Gmail, Outlook, and Yahoo are the big ones.

How much mail you send over a given period. Mailbox providers watch it closely: sudden spikes or a brand-new domain blasting hundreds of messages look like spam.

The internet's address book. DNS translates domain names like example.com into IP addresses so servers find each other, and it holds the records receivers check to verify your mail.

An email service provider: a company that gives you the infrastructure to send email at scale, handling servers, IPs, authentication, and bounce processing so you just upload and send.

Permission to email someone. It runs on a spectrum from explicit opt-in to implied, can expire, and does not cover topics they never agreed to receive.

An event or condition that automatically starts an action. In email it can fire an automated send (like re-engagement) or trip a spam or security filter.

Simple Mail Transfer Protocol: the language mail servers use to hand off and route email. Sending and receiving servers exchange commands and status codes to complete each delivery.

A database of IP addresses or domains flagged for sending spam. Mailbox providers query these in real time to accept, delay, or quietly drop your mail.

Meeting the legal and regulatory rules that govern your sending, like GDPR, CASL, and CAN-SPAM. It covers consent, data handling, and honoring opt-outs across every jurisdiction you mail.

The percentage of delivered emails where a recipient hit the spam button. Mailbox providers weigh it heavily; even 0.1% at Gmail can push you toward the spam folder.

A person who gave you permission to email them. Their engagement (opens, clicks, silence) shapes how you segment and how mailbox providers judge your sending.

The company behind Outlook, Hotmail, and Live: a major mailbox provider. It also runs SNDS for sender reputation data and Defender for email security in Microsoft 365.

The role and mailbox responsible for a domain's email problems. Providers also offer postmaster tools (like Google's) that show senders their domain reputation, spam rate, and delivery signals.

EU law in force since 2018 governing how organizations collect, use, and store EU residents' personal data. For email it requires affirmative opt-in consent before you send marketing.

The trust mailbox providers assign your domain and IP from past sending behavior. Each scores you independently, so Gmail and Outlook can rate you very differently at once.

When a recipient opts out of your emails, usually through the opt-out link. By law (CAN-SPAM) every commercial email must include a working one, so it's the channel's built-in off-switch.

The mailbox provider behind Yahoo Mail and AOL, merged since 2017. Its filters are reputation-first and engagement-heavy, and less forgiving on SPF, DKIM, and DMARC than Gmail or Outlook.

Where your email actually lands after a provider accepts it: primary inbox, a tab, or the spam folder. Delivery only means accepted; placement is what recipients actually see.

Fraudulent email impersonating a trusted brand to steal passwords, card numbers, or money. Filters treat it as criminal, not just unwanted, so detection runs harsher than spam filtering.

A do-not-send list checked before every send. It holds hard bounces, unsubscribes, complainers, and risky addresses, blocking them even if they reappear in a new import.

Microsoft's consumer mailbox provider, covering Outlook.com, Hotmail, and Live.com addresses. It hosts inboxes for end users, filters incoming mail, and offers senders SNDS for reputation data.

The trust mailbox providers attach to your sending domain, separate from IP. It follows you everywhere: change your ESP or IP and the history, good or bad, still sticks.

An email address that never belonged to a real person and never opted in. Mail reaching one signals scraped, bought, or badly maintained lists. Both pristine and recycled types exist.

The numeric identifier of the mail server sending your message. Receiving providers see it first and check its history to decide inbox or spam. IPs can be shared or dedicated.

How often you email a given subscriber. Too much drives complaints and unsubscribes; it also shapes how quickly silence counts as disengagement, so daily and monthly senders judge inactivity differently.

Splitting your list into groups so each gets relevant mail instead of one blast to everyone. It protects deliverability by keeping sends targeted at people likely to engage.

Free dashboards mailbox providers give senders to see how their mail is treated. Google Postmaster Tools is the main one, showing domain and IP reputation, spam rate, and authentication results.

A permanent delivery failure: the address doesn't exist, the domain is gone, or the mailbox was deleted. Retrying won't help, and letting them pile up damages sender reputation. Suppress immediately.

The percentage of delivered emails that logged at least one open, usually via a tracking pixel. Provider image pre-loading has made it unreliable since 2021, so treat it cautiously.

The headline shown in the inbox before an email is opened. Often the only thing a recipient sees when deciding to open, so it drives open rates.

What inbox providers think of the server your mail leaves from, built from complaints, bounces, spam-trap hits, and engagement over time. Once central, now secondary to domain reputation.

A Request for Comments: the official technical spec, published by the IETF, that defines how the internet and email work. Each gets a permanent number; updates get a new one.

Using what you know about someone to make their email genuinely relevant, from swapping a name to adapting content by behavior. Done poorly (broken merge tags), it backfires.

A Feedback Loop: the pipe a mailbox provider uses to tell you which recipient hit Report Spam, delivered as an ARF report so you can suppress them.

The percentage of your emails the receiving server returned undelivered. A closely watched list-health signal: keep total under 2% and hard bounces under 0.5%.

High-volume sending. Gmail and Yahoo count anyone over 5,000 messages a day to their users as a bulk sender, triggering SPF, DKIM, DMARC, and one-click unsubscribe rules.

A temporary delivery failure: the address exists and the server is reachable, but it couldn't accept the message now (full mailbox, overload, rate limit). Usually retried automatically.

When a send is temporarily rejected, the sending server waits and tries again on a schedule (usually exponential backoff) before eventually giving up and bouncing.

A Mail Exchange record: the DNS entry that tells other servers which mail server accepts incoming email for your domain. It governs receiving, not sending.

Hypertext Markup Language: the code that lets email carry colors, images, links, and styled buttons. Clients sanitize it on arrival, stripping scripts and forms for security.

A Mail Transfer Agent: the software that actually moves email between servers over SMTP, handling queueing, retries, and throttling. Postfix, Exim, and Sendmail are classic examples.

The computer that sends, receives, and stores email, connecting to other mail servers to hand off messages and holding yours until your client fetches them.

A cap a receiving server puts on how much mail it accepts from one IP or domain in a window. Exceed it and you get temporary deferrals (421/451), not rejections.

Google runs Gmail, the largest consumer mailbox provider, and Postmaster Tools, a free dashboard showing how Gmail views your sending domain's reputation, spam rate, and authentication.

The software you read and write email in (Outlook, Apple Mail, Thunderbird, Gmail's app). It connects to your mail server via IMAP/POP to fetch and SMTP to send.

Cold email is outreach to someone with no prior relationship who never opted in. Done well it's legitimate B2B prospecting; done badly, at volume without relevance, it's spam.

The 2003 US federal law governing commercial email. It's opt-out: you can send without consent if you use honest headers, a physical address, and honor unsubscribes within 10 business days.

Spamhaus is a nonprofit that publishes widely used blocklists (SBL, XBL, PBL, DBL, ZEN) identifying IPs and domains tied to spam. A listing can cause broad email rejection.

Machine learning: systems that learn patterns from data. Mailbox providers use it to score sender reputation and inbox placement from behavioral signals like opens, replies, and spam reports.

Uniform Resource Locator: a web address. In email, filters scrutinize your links, since long tracking URLs, shorteners, and obfuscated destinations are common spam and phishing signals.

Malware is harmful software built to infect or control a device. Email is its most common entry point, usually through malicious attachments, Office macros, or disguised links.

Email warmup gradually ramps sending volume on a new domain or IP, building sender reputation before real campaigns. It builds history from scratch but can't repair damage already done.

A contact is a single person record in your list or CRM: one email, one row. Suppression, segmentation, and engagement tracking usually happen at this individual level.

Throttling is deliberately slowing send speed. Senders throttle to look human, not bursty; receiving servers throttle to protect load and test whether they trust your reputation.

Apple runs Apple Mail and iCloud. Its Mail Privacy Protection pre-fetches images (inflating open rates), and Private Relay masks user IPs, both blurring sender tracking.

The spam or junk folder is where a mailbox provider quarantines mail it distrusts but won't reject outright. Different providers name it differently; the verdict is the same.

"RE" is the subject-line prefix marking an email as a reply to an existing thread. Using it on cold or bulk mail fakes a conversation and erodes trust.

Opt-in means a subscriber took deliberate action to sign up before you email them. Single opt-in accepts the form submission; double opt-in adds a confirmation click for quality.

Outreach is a sales engagement platform that sequences and automates outbound emails, calls, and follow-ups for sales teams, with reply detection to pause sequences when prospects respond.

Microsoft's free reporting portal for senders. Connect your sending IPs to see how Outlook and Hotmail rate your reputation: color-coded status, complaint rates, and volume data.

Deliverability company behind Sender Score (a 0-100 IP reputation rating) and paid sender certification, formerly Return Path, that mailbox providers weigh when filtering your mail.

The inspection receiving filters and ESPs run on a message before delivery: scanning links, content, and sender reputation to decide inbox, spam, or block.

The SMTP command that sets an email's envelope sender (return-path), where bounces go. It's separate from the visible From header and is what SPF checks.

Transport Layer Security: the protocol that encrypts email in transit between mail servers, so intercepted messages read as gibberish. Usually negotiated via STARTTLS, though it can silently downgrade.

Share of recipients who clicked a link in your email. Read it with caution: security scanners inflate it with automated clicks, and privacy tools hide real ones.

Sending bulk promotional or relationship-building email to people who opted in. Often cited as the highest-ROI channel, though attribution numbers like 42:1 are messier than they sound.

A general-purpose DNS record holding text data. In email it's the container for your SPF, DKIM keys, DMARC policy, BIMI, and domain-verification strings.

Email sent in bulk on your schedule to promote, educate, or keep subscribers engaged. Unlike transactional mail, recipients opted in, so consent and list quality drive deliverability.

Application Programming Interface: a defined way for software to talk to software. In email, apps use ESP APIs to send mail, pull engagement data, or validate addresses.

A DNS record that names the authoritative nameservers for a domain: the servers that answer every lookup for it, including your email records.

The ongoing practice of keeping your list accurate and permission-based: removing hard bounces, suppressing repeat soft bounces, and re-engaging or dropping subscribers who've gone quiet.

A DNS record that tells other servers where to deliver mail for your domain. It governs receiving only: sending is controlled by SPF, DKIM, and DMARC.

Canada's Anti-Spam Legislation, enforced since 2014. One of the strictest laws: it requires prior consent (express or time-limited implied) before you send commercial email to Canadians.

Short for suppression list: the running record of addresses you must never email again (hard bounces, unsubscribes, complaints). It protects your reputation and satisfies opt-out rules.

Processing that happens instantly, the moment it's triggered. In email, real-time validation checks an address during signup, versus bulk validation auditing a whole list at once.

Email sent business-to-business, targeting people at their work addresses. It runs under different laws and benchmarks than consumer email, and often hits security gateways and low-engagement role accounts.

An email security company selling cloud and on-premise filtering appliances. It also operates the free Barracuda Reputation Block List, which many enterprises use to filter inbound mail.

An email triggered by one person's action, delivering information they need right then: order confirmations, receipts, password resets. It's one-to-one, and largely exempt from CAN-SPAM's marketing rules.

A consent model where you may email people by default until they act to stop. Legal under CAN-SPAM, but weak: low engagement drags down inbox placement.

The domain in your From header: what recipients see and what mailbox providers authenticate against with SPF, DKIM, and DMARC. Your sender reputation builds on it.

Forging an email's sender so it looks like it comes from someone else, usually a trusted brand. SPF alone won't stop it because it ignores the visible From address.

Internet Service Provider. In email it can mean the network carrier (Comcast, BT) or, loosely, the mailbox provider (Gmail, Yahoo) that decides where your mail lands.

Apple Mail Privacy Protection. It pre-fetches email images through Apple's servers after delivery, firing your tracking pixel whether or not the recipient opened. This inflates and blurs open rates.

Where outbound messages wait when they can't send instantly: throttled, deferred, or retrying after a soft bounce. Your MTA or ESP holds and re-attempts them over 24-72 hours.

When a receiving server tells your IP to slow down and refuses more mail for a while, usually via 421 or 451 codes. A pause, not a rejection.

A holding area where a security gateway keeps suspect mail out of the inbox: not delivered, not bounced. Also the DMARC policy (p=quarantine) that routes failing mail to spam.

Customer Relationship Management: the database of who your contacts are, their history, and lifecycle stage. Separate from your ESP, which does the actual sending; the two usually sync.

Brand Indicators for Message Identification. It displays your verified logo next to authenticated emails in supported inboxes, and requires DMARC at enforcement (p=quarantine or p=reject) to work.

Messaging, Malware and Mobile Anti-Abuse Working Group: a nonprofit where mailbox providers, ESPs, and security firms set anti-abuse standards and publish sender best practices like confirmed opt-in.

How an email client turns your HTML into what recipients see. Each client uses a different engine, so the same email can render cleanly in one and break in another.

Machine learning that mailbox providers use to decide inbox placement, weighing engagement patterns and reputation signals instead of fixed content rules. It adapts faster than any hand-written filter.

Email addresses that either break RFC 5322 syntax rules or point to a mailbox that doesn't exist. Send to them and they hard bounce as undeliverable.

A mailbox provider's trust rating for your domain or IP, built from spam complaints, engagement, and authentication history. Often shown as a band (High to Bad), not a public number.

A named prefix under your root domain, like mail.yourbrand.com. Senders use separate subdomains to build independent reputations so one mail stream can't drag down another.

A query to the Domain Name System that resolves a name into records like MX or IP. Email depends on it; if the lookup fails, mail can't be delivered.

A general-purpose DNS record that stores free text against a domain. SPF, DKIM, and DMARC all live in TXT records because DNS never got a dedicated type for them.

A prospect who has shown some interest in your business but hasn't bought yet. You nurture and score leads to decide when and how to sell to them.

A DNS record that maps a domain or hostname to an IPv4 address. One of the oldest record types, it points names at the machines behind them.

The organization a contact belongs to. In B2B email, company traits like size, industry, and revenue drive segmentation, and mailbox providers link related company domains into one reputation family.

Google's free dashboard showing how Gmail sees your sending: domain and IP reputation, spam rate, and authentication pass rates. Your main window into Gmail deliverability.

Microsoft's free, IP-based reporting portal showing how Outlook, Hotmail, and Live filters see your sending: color-coded reputation status, complaint rates, and volume. It's a visibility tool, not a blocklist.

A permanent SMTP rejection meaning the mailbox is unavailable or the address doesn't exist. It's the most common hard bounce, and retrying won't help.

Send to a fixed set of test inboxes across providers to see where mail lands: inbox or spam. Seeds lack engagement history, so results only approximate reality.

A tiny invisible image in an email's HTML. When the client loads it, it pings the sender's server and logs an open. Image blocking makes open counts unreliable.