How to verify an email list before you send a cold campaign

Verifying a list is the cheapest protection you can buy for your domains. Bad lists are one of the most common reasons I've seen campaigns fail. It only takes a few minutes, and a campaign without it can damage domains that took weeks to get ready.

Why it matters so much

When you email an address that doesn't exist, it bounces. Filters read a high bounce rate as a sign you don't know who you're emailing, which is what a spammer looks like. Keep bounces under 2%. Above that, stop and clean the list before doing anything else.

What a verifier actually checks

• Format: whether it's a properly formed address.
• Domain: whether the domain exists and has a mail server.
• Mailbox: whether the mail server says the address exists, without sending an email.
• Risk flags: disposable addresses, role addresses like info@ and known spam traps.

What the results mean

• Valid: send.
• Invalid: remove. Never send to these.
• Catch-all or accept-all: the domain accepts everything, so the verifier can't tell. Decide on a rule. If you try them, use a small batch and watch the bounces.
• Risky or unknown: the check couldn't get a clear answer. Leave them out, or retest later.
• Disposable: remove.

The steps I'd follow

1. Clean the file. Remove duplicates and fix obvious problems with the CSV cleaner.
2. Remove opt-outs and past bounces. The suppression builder handles this.
3. Verify the whole list. For a single address, the free email validator is enough for a basic check.
4. Send only valid addresses first. Add catch-alls later, in small batches, if your bounces stay low.
5. Watch the first few hundred sends. If bounces climb over 2%, stop.

When to verify

Do it right before sending, not when you build the list. People change jobs and companies close, so a list decays while it waits. If a list has been sitting around for a few months, verify it again.

Verification doesn't catch everything

A valid address only means the mailbox exists. It says nothing about whether the person is the right one, or whether they'll want your email. The other half is targeting, which is covered in how to build a B2B lead list. Also, some mail servers won't give a true answer, so a small number of bounces is normal, even from a verified list.

Can you do it for free?

For a handful of addresses, yes. For thousands, you'll want a proper verifier, because the mailbox check takes real infrastructure. Compare the price of one with the time needed to replace a burned domain and warm up its mailboxes.

mailzy's email verifier checks lists and comes with 100 free checks. Bounce Guard pauses a campaign by itself if its bounces climb too high, so a bad list can't do unlimited damage.