Spamhaus blocklist listings

Updated

If your sending domain appears on a Spamhaus blocklist, your emails may be rejected or filtered to spam. Learn what a listing means and how to resolve it.

Spamhaus maintains one of the most widely used blocklists on the internet. When your domain is listed on the Spamhaus Domain Block List (DBL), it can’t be verified or used for sending in Customer.io until it’s removed — and delivery to most major email providers is blocked in the meantime.

Listings can escalate quickly: a domain listing can lead to an IP listing, which can result in a broader network block. That’s why it’s important to act as soon as you become aware of one.

 When you send through Customer.io’s managed email service, we monitor our shared and dedicated IPs for blocklist activity. Domain-level listings (DBL), however, are tied to your sending domain and are your responsibility to monitor and resolve.

What caused this?

Most Spamhaus listings are the result of a spam trap hit or a list bomb attack — meaning your domain was likely targeted rather than intentionally misused. That said, the listing still needs to be resolved before your domain can be re-enabled.

Resolve a Spamhaus listing

Step 1: Check your listing and request removal

Start at check.spamhaus.org. Enter your domain and Spamhaus will show you:

  • The reason for the listing
  • The specific steps or dispute process required for removal

This is your primary resource — Spamhaus handles the entire removal process there, and their guidance will be specific to your situation.

To find your Customer.io sending domain, go to Settings > Workspace Settings > Email > Sending Domains. Click Show Records and note the hostname.

Step 2: Complete the required remediation steps

Depending on the reason for the listing, Spamhaus typically requires one or more of the following before they’ll approve a removal:

  • Add CAPTCHA to your subscription forms to prevent automated sign-ups
  • Implement confirmed opt-in (COI): send a confirmation email to new subscribers and only add contacts who explicitly confirm. See our guide to double opt-in for setup instructions.
  • Clean up your list: run re-engagement campaign and remove unengaged or invalid contacts

Step 3: Submit your removal request

Once you’ve addressed the root cause, submit your removal request through check.spamhaus.org. Spamhaus will walk you through the dispute process directly.

Spamhaus reviews removal requests manually. Processing can take anywhere from a few hours to several days. You must fix the underlying problem before requesting removal! Submitting a request without addressing the cause will likely result in a rejection.

Step 4: Re-verify your domain in Customer.io

Once Spamhaus has cleared your domain, return to Customer.io and re-verify it to resume sending. Go to Settings > Workspace Settings > Email > Sending Domains and verify your domain again.

 Need to keep sending?

Customer.io supports multiple sending domains. If your primary domain is listed, you can set up and send from an alternate domain while you resolve the listing. However, be cautious about suddenly shifting all your email volume to a domain that hasn’t been used for that purpose before — this can trigger additional deliverability issues. If you’re considering this, warm up the new domain first.

Tips for avoiding future listings

  • Use confirmed/double opt-in for all new subscribers
  • Regularly clean your list: remove contacts who haven’t engaged in 6–12 months
  • Watch for sudden, unexpected spikes in new sign-ups, which can be an early sign of a list bomb attack
  • Never purchase or import email lists from third-party sources

For more deliverability best practices, see our email deliverability guide.

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