What list bombing actually does to your account
List bombing is something very few brand owners talk about, but it affects far more ecommerce stores than you might think. Essentially, list bombing happens when automated bots flood your sign-up forms and subscribe hundreds or thousands of fake email addresses to your list.
Sometimes it happens because someone is intentionally sending malicious traffic to your site to hurt your sender reputation. Other times, it is just scrapers and rogue scripts crawling web forms. Regardless of why it happens, the outcome is the same. Your deliverability gets completely trashed.
When hundreds of fake addresses land in your account, they do not open your emails, they bounce heavily, and they drag down your engagement metrics. Mailbox providers like Gmail and Yahoo notice this immediately. That is a mistake you cannot afford to ignore.
How to spot list bombing in Klaviyo
The easiest way to spot list bombing is by looking at your list growth trends. In Klaviyo, navigate to your primary newsletter list, click into it, and open the List Growth tab. You can view this on a weekly or monthly basis.
Under normal circumstances, list growth looks relatively steady. If you suddenly see a massive, unexplained spike across a single week, alarm bells should be ringing. If you did not launch a massive paid ad campaign, get featured in major press, or publish a viral piece of content, that spike is almost certainly bot traffic.
We have worked with over 120 ecommerce and B2B brands at In-box, and sudden unexplained spikes are almost always malicious. If you do not monitor your growth reports regularly, the first symptom you will notice is a sudden drop in open rates across your entire account.
Red flags inside your subscriber profiles
If you suspect an attack, jump into your recent subscribers and analyse their profile data. The red flags are usually obvious once you look closely.
- Suspicious email addresses: Strings of random alphanumeric characters before the domain that no human would ever use.
- Mismatched geographic data: If you are a New Zealand brand selling locally, suddenly gaining hundreds of subscribers from California or Illinois is a clear warning sign.
- Zero engagement: These profiles will receive your initial campaign or welcome email and register zero opens or clicks, or bounce immediately.
Every time you send to these profiles and they bounce or sit unread, you are sending negative signals straight to the inbox providers. Your deliverability score drops (for example, sliding from a healthy 46 down to 39), which means your real customers stop seeing your emails too.
Switch to double opt-in immediately
The fastest way to shut down a list bombing attack is to change your consent settings. By default, I prefer setting all lists to single opt-in because it removes friction for genuine shoppers. But here is the thing: if your deliverability is tanking from bots, you must switch to double opt-in right away.
To do this, open your list settings in Klaviyo, head to the Consent tab, and change the opt-in process from single opt-in to double opt-in. This forces every new subscriber to click a confirmation link sent to their inbox before they are added to your active database.
Bots will not confirm their subscription. Making this single change will solve roughly 90% of your list bombing problems overnight. You can keep double opt-in enabled until your list cleans up and your open rates climb back to normal levels.
Add a bounce filter to your welcome flow
While double opt-in stops new bots from entering your list, you also need to protect your automated flows from sending to invalid addresses that slipped through earlier. You can do this with a simple flow filter on your welcome series.
Open your welcome flow and add a trigger filter with the following rule:
- What someone has or has not done: Bounced Email equals 0 times in the last 30 days.
If an email address bounces on the very first message or has bounced recently, Klaviyo immediately removes them from the rest of the flow sequence. This prevents dead addresses from repeatedly bouncing across a 3-part or 5-part welcome series, which saves your domain reputation from taking further hits.
Final Thoughts
List bombing can quietly destroy your email revenue if you leave your forms unprotected. Check your list growth reports for unexplained spikes, look out for strange subscriber profiles, and switch to double opt-in the moment you suspect bot activity. Protecting your sender reputation is always worth the minor friction of an opt-in confirmation.
Protect your account deliverability
If your open rates have dropped and you suspect bot traffic or deliverability issues are hurting your sales, we can help. Book a comprehensive account audit with our team to uncover hidden deliverability issues and get your emails back into the primary inbox.
