Cold email list checker: check leads before you upload them
A cold email list checker finds the leads that would bounce or shouldn't be emailed before you upload the file to a sending tool. Drop the CSV here: it removes bad syntax, duplicates, typo and dead domains, disposable and role addresses and anyone on your do-not-contact list, then checks 10 mailboxes live. Bounces hurt new sending domains most.
Any lead here is taken off the upload file, including the same Gmail inbox written another way.
By SimpleVerifier Team · Last updated
How do I check a lead list before uploading it to Instantly or Smartlead?
- Export the leads from wherever they came from (Apollo, Sales Navigator via an enrichment tool, a scraper, a purchased file) as CSV or .xlsx.
- Optional: paste your do-not-contact list (past hard bounces, unsubscribes, people who asked you to stop) into the remove box.
- Press Check the leads. Every column you need for personalisation is kept.
- Download the clean file for upload, and the flagged rows with the reason for each.
- The first 10 addresses get a live mailbox check. Sign in with Google to check the rest with 100 free email checks before the campaign starts.
How do I verify leads before cold emailing?
In two passes. First the free checks that need no mail server: syntax, duplicates, typo domains, domains with no mail server, disposable providers and role addresses like postmaster@ that exist to receive problem reports, not pitches. Then a mailbox check on what's left, which asks each recipient's server about the address at RCPT TO and disconnects without sending (RFC 5321).
The first pass is instant and certain. The second finds the people who left their jobs, which no format check can see. Catch-all domains are the remainder: their servers accept every address, so no verifier can confirm one person, and they're best sent in a separate, smaller batch. How many emails to verify for cold email has the arithmetic.
Do Instantly and Smartlead verify emails?
Both can verify leads inside the product (Smartlead documents how to verify a lead list), and both watch bounces after sending. Smartlead, for one, documents a bounce auto-pause that stops a campaign when bounces climb; see how Smartlead verification works. A pause protects the inbox, but the bounces that triggered it have already counted against the sending domain.
Checking the file before upload means the campaign starts from rows that passed. It also means you only pay sequencer credits, or ours, for addresses that survived the free checks.
Should I remove role addresses and free webmail from a B2B list?
Remove operations and system addresses (abuse@, postmaster@, noreply@) always; RFC 2142 defines several of them for problem reports, and mail to them reaches people paid to report abuse. Business role inboxes such as sales@ and info@ are a judgement call: at a small company they're often the owner. This checker flags the first group for removal and marks the second as kept.
Free webmail on a B2B list isn't wrong, but it's a signal the lead source guessed or scraped personal addresses. To split company and webmail addresses before deciding, use the free vs business email filter.
Check before upload, or rely on the sequencer?
| Check before upload | Rely on the sequencer | |
|---|---|---|
| Bad syntax, typo and dead domains | Removed before the first send | Bounce, then pause |
| Do-not-contact list applied | Yes, across every campaign | Per campaign or workspace |
| Closed mailboxes | Mailbox check before sending | Found by bouncing |
| Effect on a new sending domain | Starts clean | First bounces count against it |
Who uses this tool, and for what?
- Cold emailers
- Check every new lead file before upload, so a warm-up period spent building reputation isn't spent on bounces.
- Lead-gen agencies
- Run client-provided lists through the same checks before any campaign, and keep the flagged-rows file as proof of what was removed.
- Founders
- Check a first outbound list from a new domain, where the first hundred sends set the tone for the domain's reputation.
What can’t this tool tell you?
- Catch-all domains can't be confirmed by any mailbox check; plan to send them separately.
- Microsoft and Apple consumer addresses aren't probed by our checker and come back risky, not valid.
- The do-not-contact match is by address (and Gmail inbox), not by person or company.
- This checks the list, not your sending setup; check the domain with the cold email domain checker.
Common questions
- How to verify emails before cold emailing?
- Remove bad syntax, duplicates, typo and dead domains, disposable and operations role addresses first, then run a mailbox check on what's left. Send catch-all addresses separately, since no verifier can confirm them.
- How to verify emails for free?
- The checks that need no mail server are free here for the whole file. The live mailbox check covers 10 addresses without an account and 100 more with a Google sign-in, no card.
- What bounce rate is OK for cold email?
- As low as you can get it. Apollo's help centre, for example, calls a bounce rate under 2% acceptable, and sending tools such as Smartlead can pause a campaign automatically when bounces climb. Verifying before upload keeps the rate down from the first send.
- Should I verify emails from Apollo or ZoomInfo again?
- Yes for anything not marked verified and anything a few months old. Data providers' statuses are snapshots; people change jobs after the snapshot.
Sources
A tidy list can still be full of dead addresses.
Extracting, deduplicating and syntax checks all work on the text of an address. None of them can tell whether the mailbox exists. Only the server that hosts it can answer that, through an SMTP check, and that is what the paid product does.
- Upload the cleaned file as it is: CSVs up to 100,000 rows
- A real SMTP mailbox check on every row, with catch-all detection
- Small list? Starter at $9/mo covers 2,500 emails a month, cancel anytime
One plan, cancel anytime, no contract. Sign in with Google and your first 100 email checks are free, no card. The free tools stay free either way.