# Email verification glossary URL: https://email-validation.org/glossary/ Updated: 2026-10-20 > Documentation index: https://email-validation.org/llms.txt Plain-English definitions of the terms used in email verification and deliverability. Each term has its own anchor, so you can link to it directly. ## Email validation Checking whether an email address is well formed and can receive mail. On this site it means the same as email verification: syntax, domain, mail records and, where possible, the mailbox itself. ## Email verification Confirming that an address can receive mail without sending a message to it. See [what email verification is](https://email-validation.org/guides/what-is-email-verification/). ## Syntax check The first step of a check: is the text a well-formed address, with a local part, an `@` and a valid domain? Malformed input fails here and costs nothing. ## Domain check Looking up a domain in DNS to see whether it exists and publishes the records needed to receive mail. ## MX record A DNS record that names the server (mail exchanger) that accepts mail for a domain, with a priority. A domain without MX records usually cannot receive mail. ## SMTP Simple Mail Transfer Protocol, the language mail servers use to pass messages to each other, defined in [RFC 5321](https://www.rfc-editor.org/rfc/rfc5321). ## SMTP check Starting an SMTP conversation with a domain's mail server and asking whether it accepts mail for an address, then stopping before any message is sent. See [verify an email without sending one](https://email-validation.org/guides/verify-email-without-sending/). ## Catch-all domain A domain whose mail server accepts mail for every address, existing or not, so individual mailboxes cannot be confirmed. Also called accept-all. See [catch-all emails explained](https://email-validation.org/guides/catch-all-emails/). ## Role address An address that belongs to a function or team rather than one person, such as `info@`, `sales@` or `support@`. ## Disposable email address An address from a throwaway inbox service, usually abandoned within hours or days. Often used to grab a sign-up reward without sharing a real address. ## Free email provider A service that gives anyone a personal mailbox at no cost, such as the large webmail providers. Addresses there are personal rather than company addresses. ## Alias address An address that forwards mail to another mailbox. Mail sent to a working alias is delivered. ## Typo domain A domain that looks like a misspelling of a common mail provider, such as a missing or swapped letter. A check can suggest the likely intended address. ## AI agent mailbox A mailbox that appears to be read and answered by an automated AI agent rather than a person. ## Spam trap An address kept only to catch senders who mail without consent. Pristine traps were never used by a person; recycled traps are old addresses that were reactivated after being abandoned. Mailing either harms your sender reputation. ## Complainer A person who often marks legitimate mail as spam. Their complaints count against senders with mailbox providers. ## Suppression list A list of addresses you must not mail, such as hard bounces, unsubscribes and complainers. Shared do-not-contact lists exist too. ## Greylisting An anti-spam technique where a mail server temporarily refuses mail from an unfamiliar sender and accepts it on a later retry. It is a common reason for an `unknown` result. ## Mailbox full A mailbox that is over its storage limit and refuses new mail until its owner makes room. ## Hard bounce A permanent delivery failure, such as an address or domain that does not exist. Remove the address. ## Soft bounce A temporary delivery failure, such as a full mailbox or a server that is down. Addresses that soft-bounce repeatedly should be treated like hard bounces. ## Bounce rate The share of sent messages that bounced. See [how to reduce your bounce rate](https://email-validation.org/guides/reduce-email-bounce-rate/). ## Sender reputation How mailbox providers judge a sending domain or IP address, based on bounces, complaints, spam traps and engagement. A poor reputation sends mail to spam folders. ## Deliverability How much of your mail reaches the inbox rather than the spam folder or nowhere at all. ## List hygiene Keeping a mailing list healthy by verifying addresses, suppressing bounces and removing inactive or risky contacts. See [how to clean an email list](https://email-validation.org/guides/email-list-cleaning/). ## Double opt-in Asking new subscribers to confirm their address by clicking a link in a confirmation email. It proves the person controls the mailbox, which verification alone cannot. ## SPF, DKIM and DMARC Three DNS-based standards that let receivers check that mail claiming to come from your domain really does: SPF lists the servers allowed to send, DKIM signs each message, and DMARC tells receivers what to do when checks fail. ## Unknown result A check that could not reach a definite answer, for example because the mail server did not respond in time. Unknown results are free with email-validation.org. See [the unknown status](https://email-validation.org/docs/statuses/unknown/). ## Credit The unit you pay with: one credit per result with the status deliverable, undeliverable or risky. See [credits and billing](https://email-validation.org/docs/credits-and-billing/). ## API key A secret that identifies your account to the API. Send it in the `Authorization` header, never in a URL. See [authentication](https://email-validation.org/docs/authentication/). ## Rate limit The most requests an account or key may send per second. See [rate limits](https://email-validation.org/docs/rate-limits/). ## Webhook An HTTP request we send to your server when something happens, such as a bulk job finishing. See [webhooks](https://email-validation.org/docs/webhooks/). ## OpenAPI A standard, machine-readable way to describe an HTTP API. Our [API reference](https://email-validation.org/docs/api-reference/) is generated from our OpenAPI 3.1 document. ## MCP (Model Context Protocol) An open protocol that lets AI assistants and agents use external tools. Our [MCP server](https://email-validation.org/docs/mcp/) lets them validate addresses with your API key.