Email verification
Email verification is the process of checking whether an email address is valid and safe to send to — confirming the syntax, domain and mailbox exist and aren't a known trap or disposable address — before you add it to a campaign. It is the single most effective way to control bounce rate.
How it works
A verifier checks format, validates the domain's MX records, and probes the mailbox (without sending) to classify the address as valid, invalid, catch-all or risky.
Why it matters
Sending to unverified lists spikes bounces and spam-trap hits, which damages sender reputation across all your mail. Verifying first is far cheaper than recovering a burned domain.
What happens during an SMTP mailbox check?
The verifier looks up the domain's MX records to find its mail server, opens a connection, and goes through the opening of a normal delivery: a greeting, a MAIL FROM line naming a sender, then a RCPT TO line naming the address being checked. It reads the server's answer to that last line and hangs up before any message is sent. A 250 response means the server would accept mail for that recipient; a 550 with a no-such-user message means it would not.
Some servers make the answer ambiguous on purpose. Greylisting returns a temporary 4xx code to unfamiliar senders, so the result is unknown until a retry. Servers that accept every recipient give a 250 whether the mailbox exists or not, which is how a catch-all email domain looks from the outside.
What should you do with each verification result?
Treat each category as a sending decision. Valid addresses go into campaigns. Invalid ones go straight to suppression. Catch-all and risky results go into their own segment with lower volume. Unknown results, often caused by greylisting or a timeout, get checked again a day later. Disposable and role addresses are a policy call, and most B2B teams exclude them.
In a hypothetical 5,000-contact import that returns 4,100 valid, 350 invalid, 450 catch-all and 100 unknown, the main campaign gets 4,100 addresses. The 450 go into a small test, the 100 are rechecked, and the 350 never receive a message. Sending all 5,000 at once would have put 350 certain bounces into the first days of the campaign.
When should verification run: at capture, at import, or before sending?
Each point catches something different. A real-time check on a web form stops typos while the person is still on the page. A batch check at import clears a purchased or exported list before it touches a sequence. A recheck before sending catches addresses that have died since the import, which matters for any segment older than a few months.
Plan capacity around your daily allowance. Autocloz verifies addresses before sending and includes 1,000 verifications a day on the Free plan, so a 5,000-contact import clears in five days at that rate; importing in daily batches keeps campaigns fed without a backlog. For a quick single lookup, the free email checker runs in the browser.
How is verification different from validation or enrichment?
Syntax validation only confirms the address is well formed, so [email protected] passes even though the domain is probably a typo. Lead enrichment finds or guesses an address, often from a naming pattern such as first.last. Verification is the step that asks the receiving server whether that address will accept mail. A guessed address that has not been verified is the most common source of avoidable bounces, which feed straight into your email bounce rate.
How Autocloz handles it
Autocloz includes email verification (1,000/day on the free plan, more on paid) and verifies addresses before sending, auto-suppressing invalid ones so a bad import can't tank your reputation.
Free tools for this
No signup required — they run in your browser.
FAQ
How accurate is email verification?
For most mailboxes it reliably flags invalid and disposable addresses. Catch-all domains accept any address, so verifiers mark them 'risky' rather than valid — send to those cautiously.
Related terms
Email bounce rate is the percentage of sent emails that are returned undelivered. Hard bounces are permanent (invalid or non-existent address); soft bounces are temporary (full mailbox, server down). A high bounce rate signals a poor list and damages sender reputation.
A spam trap is an email address operated by mailbox providers and blocklist operators specifically to catch senders with poor list hygiene. There's no real person behind it, so any mail it receives indicates you're emailing addresses you didn't earn permission to contact.
A hard bounce is a permanent email delivery failure — the address is invalid, the domain doesn't exist, or the mailbox was deleted — and should be suppressed immediately. A soft bounce is a temporary failure — a full mailbox, a message too large, or a server briefly down — which may succeed on a later retry.
List hygiene is the ongoing practice of keeping an email list clean and accurate — verifying addresses, removing invalid and hard-bounced contacts, suppressing unsubscribes and long-term non-engagers, and avoiding purchased or scraped data. Good hygiene protects deliverability and keeps sender reputation healthy.
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