Is email verification 100% accurate?

No. It cannot confirm mailboxes on catch-all domains, some servers refuse to answer, and an address can be closed after it was checked. It sharply reduces bounces but does not eliminate them.

Does verifying an email send a message to the person?

No. The verifier opens a conversation with the mail server and stops before any message content is sent. The recipient sees nothing.

How often should I re-verify a list?

Before every campaign if the list is more than a month or two old. People change jobs and work addresses are closed when they leave.

What is email verification?

How verification works

A verifier runs a series of checks and stops at the first failure.

Syntax. Is it shaped like an address? jane.doe@acme fails here.
Domain. Does the domain exist and publish MX records saying which servers accept its mail? No MX, no delivery.
Mailbox. The verifier connects to that server over SMTP and goes as far as the RCPT TO command, which names the recipient. Under RFC 5321 the server replies with a code: a 2xx code means it will accept mail for that address, a 5xx code is a permanent refusal (often "user unknown"), and a 4xx code means try later.
Risk flags. Role addresses (info@, sales@), disposable domains and known spam traps are marked even if they exist.

The verifier then disconnects without sending a message.

What the results mean

Result	Meaning	Send?
Valid	Server confirmed the mailbox	Yes
Invalid	Server refused the mailbox, or no mail server	Never
Catch-all	Server accepts every address, so it proved nothing	Only with care
Unknown	Server timed out or blocked the check	Re-check later
Role or disposable	Exists, but not a person or not permanent	Usually no

A worked example

Say a waterfall finds 500 addresses. Verification returns 380 valid, 45 invalid, 60 catch-all and 15 unknown. Sending to all 500 would risk at least 45 hard bounces, a 9% bounce rate before any catch-alls fail. Sending only to the 380 valid ones and a small test batch of catch-alls keeps you in a range mailbox providers tolerate. The 45 invalid addresses cost nothing to drop. Sending to them would have cost reputation.

Common mistakes

Verifying once, sending months later. An address valid in January can bounce in June.
Treating catch-all as valid. See catch-all email for why it is a separate risk.
Verifying from your sending domain or IP. Many checks in a short burst can look like a dictionary attack. Use a verification service.
Paying for invalid results. Check whether your data supplier charges for addresses that fail.

Related terms

Verification protects email deliverability and is the last step of any waterfall enrichment run. Our guide on how to verify email addresses covers tools and process.

How Sluice handles it

Every address Sluice finds is verified independently, only verified addresses are billed, and if one still bounces, the cost of that lookup is refunded exactly, once. Sequences stop for any address that bounces.