Email verification glossary
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.
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.
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.
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.
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.
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.
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.
Credit
The unit you pay with: one credit per result with the status deliverable, undeliverable or risky. See 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.
Rate limit
The most requests an account or key may send per second. See rate limits.
Webhook
An HTTP request we send to your server when something happens, such as a bulk job finishing. See webhooks.
OpenAPI
A standard, machine-readable way to describe an HTTP API. Our 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 lets them validate addresses with your API key.