Email verification glossary
Email verification terms and what they mean
Look up a term from a verification result or a mail-server response. Each entry explains the mechanism, what the evidence establishes and where uncertainty remains. Start with the term you need below.
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Catch-all domain
A mail server accepts arbitrary recipients, so acceptance alone cannot establish whether a particular mailbox exists.
Greylisting
A temporary SMTP refusal asks the sender to try again. A later attempt may provide more evidence.
Spam trap
An address used to identify problematic sending practices. A verification check cannot certify that an address is not a trap.
Role account
An address for a team or function, such as support@. It can receive mail; its suitability depends on the purpose.
Disposable email address
An address at a temporary-mail provider. The domain signal concerns durability and policy, rather than proving delivery failure.
Hard bounce and soft bounce
Permanent and temporary delivery failures describe a sending attempt. Learn how they differ from verification evidence.
SMTP handshake
The greeting and commands used to ask a mail server about a recipient, stopping before message content is sent.
MX record
A DNS record that identifies a domain’s mail servers and their priority. A mail route does not prove that a mailbox exists.
Email address syntax
Address-format rules checked before network lookups. A valid format does not establish a working domain or mailbox.
DNS failure
A domain lookup may report nonexistence or a temporary resolution problem. Those outcomes require different handling.
Throttling and rate limiting
A destination limits how often probes can connect or ask about recipients. The limit does not establish mailbox existence.
Ambiguous rejection
A recipient refusal can hide mailbox information or reflect server policy. A permanent-looking reply need not justify rejection.
TLS and STARTTLS failures
An attempted STARTTLS upgrade fails before the recipient check. Missing STARTTLS support and certificate validation are separate questions.
Policy rejection
A destination refuses a session or recipient under its own rules. The refusal alone cannot establish that the mailbox is absent.
Codes that describe us, not the address
Service protections, capacity limits or incomplete work can prevent a probe. These codes explain the check, rather than the mailbox.
Protocol errors
A reply or exchange cannot be processed within the SMTP client’s protocol and resource limits. A valid negative reply is a different case.
Addresses this service will not probe
An unsupported address form or unsafe destination can stop the check by design. The limitation does not prove delivery failure.
See the evidence for an address
Create an account to run a check and read its status, recommended action and reason codes.