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Step-by-step guide to verifying a B2B email list before cold outreach: verification methods, tools compared, what Quarvio verifies, and the cost of skipping.
Ryan Mercer
SDR turned cold email consultant, 8 years outbound · Updated June 24, 2026
Last updated: June 2026 · Ryan Mercer, SDR turned cold email consultant, 8 years outbound
TL;DR — 5 things to know before reading
Email list verification is the most underinvested step in cold email infrastructure, and it is the step that causes the most recoverable damage when skipped. Across dozens of outbound program audits, the pattern repeats: excellent campaign setup, solid copy, well-warmed inboxes, and a contact list that was never verified. The result is bounce rates of 8–15%, domain reputation degradation, and months of warmup effort destroyed in days.
Verification is a solved problem. SMTP-level verification services can check a 10,000-contact list in under an hour. Quarvio-sourced contacts arrive pre-verified at order time, which eliminates the verification step entirely for fresh data. The only scenario where re-verification is needed is when a list has been held for more than 6 months before sending, because B2B addresses decay at approximately 20% per year.
This guide walks through the complete verification process: what each level checks, how to handle catch-all domains, what to do with risky contacts, and what Quarvio verifies so you know what additional steps are needed for your specific situation. The full workflow connects to how to use Instantly with Quarvio once verification is complete.
Not all verification is the same. There are three progressively more accurate levels, and the tool you use determines which level of verification you are actually performing.
Level 1: Syntax check
Checks whether the email address is formatted correctly. An address like “john@” or “john.company.com” would fail a syntax check. This is the lowest level of verification and catches only obvious formatting errors.
Most spreadsheet import tools perform at least a syntax check. It does not verify that the address exists or can accept email — it only confirms the format is valid.
Level 2: Domain check (MX record lookup)
Checks whether the domain in the email address has valid MX (Mail Exchange) records configured, meaning the domain is set up to receive email. An address at a defunct domain would fail if that domain no longer has MX records.
Domain checks catch a meaningful share of invalid addresses — specifically those at domains that have been abandoned or never configured for email. Per Mailmodo's B2B email marketing statistics, domain-level invalid addresses are more common in purchased lists from low-quality data providers.
Level 3: SMTP verification
SMTP verification connects to the recipient's mail server and asks whether the specific mailbox exists, without actually sending an email. This is the gold standard of verification. It catches invalid addresses even when the domain has valid MX records.
For example: an address at a valid company domain would pass a domain check (the domain exists and accepts email) but fail an SMTP check if the person no longer works at that company and their mailbox has been deprovisioned.
The limitation of SMTP verification is catch-all domains (discussed below). If a domain is configured to accept email to any address, SMTP verification returns “valid” even for non-existent mailboxes.
| Verification level | What it catches | What it misses |
|---|---|---|
| Syntax check | Malformed addresses | All invalid-but-formatted addresses |
| Domain check | Non-existent or expired domains | Invalid mailboxes on valid domains |
| SMTP verification | Invalid mailboxes on valid domains | Catch-all domains |
Catch-all domains are a significant challenge in B2B verification. A catch-all configuration means the mail server accepts delivery to any address at the domain — whether or not that mailbox exists. The SMTP verification step cannot distinguish a real mailbox from a non-existent one on a catch-all domain.
Catch-all domains are common among mid-sized companies that use them to prevent missed emails from address typos. They are significantly more common in certain industries (professional services, consulting, law firms) than in others.
The correct handling for catch-all addresses:
Woodpecker's 2025 cold email benchmark study data supports this segmentation approach: catch-all addresses typically show 10–20% lower open rates and higher bounce rates than verified clean addresses from the same company type.
Step 1: Export your contact list to CSV
Export the contact list from your data source to a CSV file. The minimum required column is the email address. Additional columns (name, company, title) do not affect verification but should be preserved for import to your sending platform.
Step 2: Choose your verification approach
For lists sourced from Quarvio: contacts arrive pre-verified at SMTP level. Skip to Step 5 unless the list has been held for more than 6 months.
For lists from other sources or lists older than 6 months: run through an SMTP-level verification service. The key capability to look for: the service must perform full SMTP verification (not just syntax and domain checks) and must return catch-all status separately from the valid and invalid categories.
Step 3: Upload and run verification
Upload the CSV to your chosen verification service. Most services process results in under an hour for lists under 50,000 contacts. The output should categorize each address as one of: Valid, Invalid, Catch-All/Risky, or Unknown.
Step 4: Segment the results
Create three output lists:
Never send to Invalid addresses. Never mix Invalid addresses into any sending campaign, even as a small percentage of the total.
Step 5: Add valid contacts to Instantly
Add the Valid segment to Instantly and assign to the appropriate campaign. During the upload, map the CSV columns to Instantly's lead fields (first name, last name, email, company, job title). Confirm the lead count before activating the campaign.
Step 6: Monitor bounce rate on first 100–200 sends
Even with verified contacts, monitor the bounce rate on the first 100–200 sends of any new campaign before scaling. A bounce rate above 1% on verified data signals that the data has aged significantly since verification and may need re-verification before continuing.
Contacts ordered from Quarvio go through SMTP-level verification at order time.
Quarvio verifies:
When re-verification is needed:
When re-verification is NOT needed:
For most use cases, contacts ordered from Quarvio and imported to Instantly within 30–60 days can be sent to directly without an additional verification step.
Skipping email verification is a false economy. The cost calculation:
| Scenario | Contact count | Bounce rate | Domain recovery time |
|---|---|---|---|
| Quarvio-verified, sent within 60 days | 5,000 | Under 1% | No recovery needed |
| Unverified list, clean source | 5,000 | 3–8% | 2–4 weeks warmup degradation |
| Unverified list, low-quality source | 5,000 | 10–25% | 6–12 weeks to rebuild, or domain replacement |
Domain warmup takes 4–8 weeks per Woodpecker's email warmup recovery guide. A blacklisted domain may require full replacement, which means starting the warmup process from zero on a new domain. The cost of domain replacement and warmup far exceeds any cost savings from skipping verification.
Per Google's email sender guidelines, senders who produce high bounce rates will have their emails downgraded or blocked from Gmail inboxes — a consequence that affects all campaigns from that domain, not just the campaign that triggered the issue.
| Need | Tool | Notes |
|---|---|---|
| Pre-verified B2B contacts | Quarvio | SMTP-verified at order time, catch-all flagged separately |
| Email inboxes | Inframail | Microsoft 365 inboxes with correct authentication |
| Cold email sending | Instantly | Bounce tracking per campaign, auto-pause on threshold breach |
| LinkedIn outreach | Aimfox | LinkedIn channel to same ICP contacts in parallel |
How often should I re-verify a B2B email list?
Verify immediately before any send to a list that is 6 months old or older. For lists used within 60 days of acquisition from a pre-verified source like Quarvio, re-verification is not typically necessary. For high-turnover industries such as startups, early-stage tech companies, and SDR-heavy organizations, consider re-verifying every 3–4 months regardless of data age, as role turnover is faster than industry average.
What is a catch-all domain and why does it cause problems?
A catch-all domain accepts email sent to any address at that domain, even if the specific mailbox does not exist. When you send to a catch-all domain, the mail server accepts delivery, so your sending tool does not register a bounce. But the email may be silently discarded at the server level if the mailbox does not exist. Catch-all domains appear valid in SMTP verification tools but can still have high effective non-delivery rates. Segment catch-all addresses separately and send from a secondary domain to avoid risking your primary infrastructure.
What bounce rate should I target after verifying my list?
After SMTP-level verification with a quality tool, target under 1% hard bounce rate on the first campaign send. A verified list from Quarvio sent within 60 days of order should reliably produce under 0.5% bounce rate. If post-verification bounce rate is above 2%, the verification tool may not be performing full SMTP checks, or the list has aged significantly since verification occurred.
Does Quarvio's verification guarantee deliverability for every contact?
Quarvio's 90% deliverability guarantee means that 90% or more of delivered contacts are SMTP-verified as valid mailboxes at the time of order processing. The remaining margin accounts for catch-all domains and the inherent uncertainty in contacts that can only be definitively confirmed through actual delivery. This is meaningfully higher than the industry default of unverified data, which typically shows 15–25% bounce rates on first send.
Can I verify a list myself without using a dedicated service?
You can run basic syntax and domain checks manually using spreadsheet tools or free online checkers. However, full SMTP verification requires connecting to mail servers and is not practical to run manually at scale. For lists above a few hundred contacts, a dedicated SMTP verification service is the only practical approach. For contacts sourced from Quarvio, the verification is done for you at order time, which eliminates this step entirely for fresh data.
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