What are the common signs of an email list bombing attack?

Look for sudden spikes in sign-ups, suspicious data in form fields (like gibberish names), multiple sign-ups from the same IP address, or sign-ups directly to your API instead of forms. Analyzing timestamps can also reveal concentrated bursts of activity.

How do I remove email addresses that were submitted via list bombing?

You can remove them by segmenting your list based on the suspicious patterns identified during the attack, such as IP address, sign-up date/time, and unusual name fields. Automated tools within your email service provider or CRM can help with bulk removal after identification.

What preventative measures can I take to stop list bombing?

Double Opt-In (DOI) is highly effective as it requires users to confirm their subscription. CAPTCHA challenges (like reCAPTCHA) and hidden honeypot fields on your forms can also prevent bots from successfully submitting email addresses. Additionally, consider disallowing sign-ups from IP addresses known for botnet activity via reputable blocklists.

What impact does list bombing have on email deliverability and sender reputation?

List bombing can significantly harm your sender reputation, leading to higher bounce rates, increased spam complaints, and a greater chance of your legitimate emails landing in the spam folder or on an email blocklist. This can negatively impact your overall email deliverability.

How can I identify and remove email addresses submitted via list bombing? - Technical - Email deliverability - Knowledge base

Email list bombing, also known as subscription bombing, is a malicious attack where an attacker subscribes a single email address (or a small set) to hundreds or thousands of online newsletters, services, and websites. The goal is often to overwhelm the victim's inbox, making it difficult for them to find legitimate emails, or to hide other malicious activities, such as account compromises. For businesses, it means a deluge of fake or unwanted sign-ups, which can harm your sender reputation and inflate your subscriber lists with junk data.

Dealing with list bombing requires a two-pronged approach: robust preventative measures to stop these fake subscriptions at the source and effective post-attack strategies to identify and remove email addresses that have slipped through. Without proper defenses, these attacks can lead to increased bounce rates, spam complaints, and eventually, your emails landing in the spam folder or on an email blocklist (or blacklist).

Preventing list bombing before it starts

The most effective defense against list bombing is to prevent fake sign-ups from occurring in the first place. Implementing strong preventative measures on your signup forms and throughout your email collection process is crucial.

One of the simplest yet most effective methods is Double Opt-In (DOI). With DOI, after someone submits their email, they receive a confirmation email with a link they must click to verify their subscription. This prevents bots from flooding your list with unverified addresses, as they typically won't be able to click the confirmation link. You can also review how to prevent fake email registrations and other related preventative measures.

CAPTCHA challenges, such as reCAPTCHA, are another common and useful tool. They introduce a challenge that is easy for humans but difficult for bots, effectively filtering out automated sign-ups. While not foolproof, they significantly reduce the volume of bot traffic.

Honeypot fields offer a more subtle approach. These are hidden fields in your forms that are invisible to human users but are filled in by bots, flagging the submission as suspicious. This method can catch many automated scripts without inconveniencing legitimate users. More information on how this method can prevent list bombing is available from Klaviyo's guide to understanding list bombing.

CAPTCHA challenges

User experience: May introduce friction for human users, potentially reducing conversion rates.
Effectiveness: Highly effective against a wide range of bots, especially when using advanced versions like invisible reCAPTCHA.

Honeypot fields

User experience: Seamless and invisible to legitimate users, no added friction.
Effectiveness: Catches simpler bots that automatically fill all form fields. More sophisticated bots might bypass it.

Identifying a list bombing attack in progress

Even with strong preventative measures, some malicious sign-ups might still get through. Identifying an ongoing list bomb (or blacklist) attack quickly is key to minimizing damage. One of the first indicators is a sudden, inexplicable surge in new sign-ups, far beyond your typical rates.

Beyond raw numbers, examine the data patterns associated with these new sign-ups. Bots often leave distinct footprints. Look for unusual or similar patterns in the first and last names, such as random strings of characters, hex codes, or names that don't appear legitimate. Also, check the source IP addresses for unusual concentrations or IPs known for bot activity. You might find multiple sign-ups originating from the same IP or a small range of IPs.

Another red flag is email addresses being submitted directly to your back-end APIs rather than via your website forms. This indicates a programmatic bypass of your front-end defenses. Examining the user-agent strings or other data POSTed during the submission can also reveal anomalies. Pay close attention to timestamps, as list bombing attacks often occur in short, intense bursts. Detecting these patterns allows you to identify suspicious activity efficiently. Consider reviewing how to identify and prevent suspicious email addresses.

For ongoing prevention, consider integrating checks against reputable IP blocklists (or blacklists). Services like Spamhaus's XBL (Exploits Blocklist) can identify IPs associated with botnets or compromised machines. By disallowing sign-ups from IPs on such blocklists, you can preemptively block a significant portion of bot traffic. Be cautious, however, not to use broad blocklists like PBL (Policy Block List) that might inadvertently block legitimate users. You can also monitor your own IP for blocklist monitoring.

Signs of a list bombing attack

Sudden spikes: An abrupt, uncharacteristic increase in new email sign-ups.
Suspicious data: Gibberish, hex codes, or repetitive patterns in name fields.
Concentrated IPs: Many sign-ups from the same or a small range of IP addresses.
API submissions: Sign-ups bypassing your public forms, going directly to your backend.
Timeframe: Large volumes of sign-ups occurring within a very short period.

Removing affected email addresses effectively

Once you've identified a list bombing attack, swift action is necessary to remove the compromised email addresses. Leaving these invalid or fake addresses on your list can severely damage your sender reputation, leading to poor deliverability and potentially getting your IP or domain on an email blacklist (or blocklist).

Your best bet for removal after the fact is to leverage your own subscription audit data. This means thoroughly reviewing recent sign-ups for the patterns mentioned earlier, such as identical sign-up timestamps, suspicious first/last names (e.g., those containing hex codes), or multiple sign-ups originating from the same IP address. Segment your list based on these patterns to isolate and then remove the suspicious contacts. You can learn more about how to remove bad email addresses.

For ongoing email list hygiene, regular auditing of new subscribers is essential. Implement automated processes to flag or quarantine suspicious sign-ups before they affect your sending. This proactive data cleaning helps maintain a healthy email list and protects your reputation. Many email service providers offer tools to help with email validation and list maintenance.

Data Point	Suspicious Indicators
Timestamp	Multiple sign-ups at exact or near-exact same times.
IP Address	Many sign-ups from a single IP or a narrow range of IPs (often data centers or VPNs).
Name Fields	Random characters, unusual patterns, or hex codes in first/last name fields.
User-Agent	Generic, missing, or bot-like user-agent strings.
Referral Source	Direct API submissions instead of form-based referrals.

Protecting your sender reputation long term

Ultimately, the effectiveness of your email program hinges on maintaining a pristine sender reputation. List bombing attacks can severely tarnish this, leading to emails being flagged as spam, deferred, or outright rejected by mailbox providers. A clean list, free from fake or compromised addresses, signals to ISPs (Internet Service Providers) that you are a legitimate sender.

Beyond list hygiene, ensure your email authentication protocols are robust. Implementing SPF, DKIM, and DMARC helps mailbox providers verify that your emails are legitimate and haven't been spoofed. These measures, combined with vigilance against list bombing, create a strong foundation for excellent email deliverability.

Views from the trenches

Best practices

Always implement Double Opt-In (DOI) for new subscriptions to verify legitimate users.

Use a combination of CAPTCHA and honeypot fields on all email signup forms.

Regularly monitor your new sign-ups for unusual patterns in data, IP addresses, and timestamps.

Segment and remove suspicious email addresses promptly to protect your sender reputation.

Disallow signups from IPs listed on reputable botnet blocklists like Spamhaus XBL.

Common pitfalls

Relying solely on third-party email verification APIs after submission, as they may lack crucial context.

Delaying the removal of list-bombed email addresses, which can lead to higher bounce rates and spam complaints.

Using overly aggressive IP blocklists (e.g., Spamhaus PBL) that might inadvertently block legitimate users.

Neglecting to analyze the source IP and other submission data, missing key bot fingerprints.

Failing to implement a multi-layered defense strategy, making your forms vulnerable.

Expert tips

Consider hidden fields for less sophisticated bots. If filled, flag the entry as suspect.

Analyze historical data for unique patterns during attacks, such as unusual user-agent strings.

If an email verification service has a JavaScript front-end integration, it might provide more data about the source IP and browser, aiding in bot detection.

Keep an eye on programmatic bypass attempts directly to your backend APIs.

Regularly audit your subscription data for consistency and anomalies.

Expert view

Expert from Email Geeks says if you are calling list verification via a traditional API after submission, it is often too late because the platform lacks source IP or browser information to fingerprint bot traffic.

2024-09-12 - Email Geeks

Marketer view

Marketer from Email Geeks says that your own subscription audit data is the best resource for removing fraudulent sign-ups after a list bombing incident.

2024-09-12 - Email Geeks

Navigating the aftermath: A summary

Dealing with email addresses submitted via list bombing requires a comprehensive strategy. Proactive measures, such as Double Opt-In, CAPTCHA, and honeypot fields, are your first line of defense, significantly reducing the influx of fake sign-ups. When attacks do occur, quickly identifying suspicious patterns in your signup data—from IP addresses to submission timestamps and unusual name formats—is critical.

Once identified, prompt removal of these compromised addresses is essential to protect your sender reputation and ensure high email deliverability. By combining robust prevention with vigilant monitoring and swift remediation, you can effectively combat list bombing and maintain a healthy, engaged email list.