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Posted At: Mar 24, 2026 - 794 Views

How to Verify an Email List Before Importing It into Brevo or Klaviyo

You're migrating from an old sending tool, reviving a dormant contact database, or preparing your first campaign on Brevo or Klaviyo. Before you import your CSV file, there's one step that often gets overlooked — even though both platforms recommend it: checking that the addresses on your list are actually usable.

Here's why it matters, and how to do it in practice.

What Brevo and Klaviyo recommend before any import

Klaviyo strongly recommends importing clean lists and sending to an engaged audience first. If you import an old list without sorting it first, the deliverability of your first sends may suffer.

Brevo, for its part, asks you to confirm that the imported list contains only legitimate, properly prepared contacts before the import is validated.

Both platforms start from the same principle: a clean list going in protects your deliverability from the very first send.

Why an unverified list can hurt you from the first send

Klaviyo warns that if your list contains profiles that have not explicitly agreed to receive marketing emails, this can hurt your deliverability.

Beyond consent, the practical problem is the technical quality of the addresses themselves. A list recovered from an old CRM, exported from an Excel file or built up over time without verification often contains:

  • addresses with typos (non-existent domain, missing @ sign)
  • disposable or temporary addresses used to get a discount voucher
  • mailboxes that have been deactivated or abandoned since sign-up
  • role addresses (contact@, info@, support@) that are poorly suited to marketing campaigns

These addresses can pass the most basic format checks — and then generate bounces on the very first send.

What Brevo and Klaviyo check (and what they don't)

Brevo accepts CSV, XLSX and TXT files. During import, the platform mainly flags format, structure or data errors.

Klaviyo also flags the most common import issues, such as invalid characters in email addresses or missing required fields.

In short: at import time, Brevo and Klaviyo mainly check file quality and data format. That doesn't replace a dedicated technical check that the mailbox actually exists before you send.

How to verify your list before import with GTMail-Verif

GTMail-Verif is a French email verification tool designed to clean a list before sending. It accepts CSV and Excel files directly from the interface, with no API required.

For each address, it checks:

  • syntax and format (address structure, domain)
  • the MX record (can the domain receive email?)
  • whether the mailbox actually exists, using a non-intrusive SMTP ping
  • whether the address is disposable or temporary
  • role addresses (admin@, support@, contact@)
  • whether there is a catch-all server

At the end of the analysis, you get a report with the status of each address (valid, invalid, risky, disposable) and you download a clean list ready to import into Brevo or Klaviyo.

Pricing: free access with 50 verifications per day — Pro plan at €29/month for 10,000 verifications per month.

The right order of steps before a Brevo or Klaviyo import

  1. Export your list from your old tool, CRM or source file in CSV or XLSX format
  2. Verify the addresses with GTMail-Verif — status, score and reasons for each contact
  3. Remove or isolate invalid, disposable and risky addresses
  4. Import the clean list into Brevo or Klaviyo
  5. Import or blocklist the unsubscribes and hard bounces from your history before sending

Klaviyo recommends importing your unsubscribe and bounce history into the suppressed profiles list before the first send, so you don't contact those profiles again and protect your deliverability from the start.

In summary

Brevo and Klaviyo are powerful tools for managing and sending your campaigns — but neither one replaces a technical check of your addresses before import. A list verified upfront protects your sender reputation from the very first send, avoids unnecessary bounces and ensures your statistics reflect reality.

→ Verify and clean your list before import with GTMail-Verif.

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