How can I prevent list bombing?

In the world of email marketing, protecting your contact list is crucial to ensure strong deliverability and maintain your sender reputation. However, a growing and often underestimated threat is disrupting this balance: list bombing. This type of attack floods your mailing list with unsolicited or invalid email addresses. The consequences can be severe, from plummeting open rates to your domain being blacklisted. In this article, we'll explain what list bombing is, how to identify it, the impact it can have, and how to effectively protect yourself using the tools available in SystemeScale.

1. What is list bombing?

List bombing is a malicious automated attack where bots exploit vulnerable subscription forms by submitting thousands of fake opt-ins. This rapidly fills your contact list with invalid email addresses or real addresses belonging to people who never consented to receive communications from you.

2. How does list bombing affect you?

List bombing is a serious security issue that can severely damage your technical email deliverability framework in several ways:

• Hard Bounces & Spam Complaints: Your campaigns will generate massive bounce rates and spam reports, instantly destroying your sender reputation.
• Lower Open Rates: Since the bots or unwilling recipients won't engage with your content, your overall open metrics will plummet.
• Spam Trap Risks: Bot lists often contain known spam traps designed by ISPs to catch irresponsible senders.
• Blacklisting: Your sender domain name may be permanently blacklisted, meaning all your emails will default to spam folders or get blocked by recipient servers entirely.

3. How to identify a list bombing attack?

You can usually spot a list bombing event by monitoring your metrics for these common indicators:

• A sudden, massive, and unexpected spike in new subscribers.
• All the new email addresses came through the exact same signup form.
• The influx of emails occurred during a specific, very short timeframe.
• An unusually high bounce and/or spam rate during that specific timeframe compared to your historical averages.
• The captured contacts do not contain real data; they often feature randomly generated, gibberish first and last names.

4. How can I prevent list bombing?

To prevent list bombing and block malicious bot attacks, you must implement these two primary security procedures on all your active forms:

Enable reCAPTCHA

Enabling reCAPTCHA prevents bots from successfully submitting your forms. ReCAPTCHA is a security gate that requires subscribers to perform a short task to verify that they are real human users.
Learn more: How to add a CAPTCHA to your forms

Activate Double Opt-In

Double opt-in is a strict verification process where a new subscriber must confirm their subscription before being officially added to your database. This feature requires new contacts to verify their email by clicking a link sent to their inbox. It automatically excludes unverified bot addresses, drastically increasing the quality of your list.
Learn more: How to set up double opt-in

5. What to do if you have already been a victim of List Bombing?

Scenario A: The attack targeted a SystemeScale registration form

Scenario B: The attack targeted an inline or pop-up form embedded on an external site

If you need assistance analyzing an attack or navigating the cleanup process, you can reach out to our deliverability team at any time. We will gladly analyze the situation and guide you through every step.