How to identify and remove bot-generated spam email addresses from contact lists and prevent future bot sign-ups?
Michael Ko
Co-founder & CEO, Suped
Published 21 Jul 2025
Updated 21 Jul 2025
5 min read
Summary
Bot-generated spam email addresses are a significant challenge for email marketers, leading to inflated lists, inaccurate analytics, and potential deliverability issues. These bots often sign up for email lists using real or randomly generated email addresses, making them hard to distinguish from legitimate subscribers. Addressing this requires a dual approach: identifying and removing existing bot sign-ups and implementing robust preventative measures to stop future attacks. Effective list hygiene is crucial for maintaining a healthy sender reputation and ensuring your messages reach engaged recipients. Preventing fake email registrations and list bombing is key to email marketing success.
Key findings
Identification challenges: Bots often use normal-looking email addresses, making it difficult to filter them based on email format alone. Advanced data points are required.
Proactive prevention: Implementing tools like CAPTCHA, reCAPTCHA, or hidden form fields (honeypots) can significantly reduce bot sign-ups.
Post-breach cleanup: For existing bot-infested lists, leveraging signup data such as IP addresses, referer information, and browser identifiers can help in identifying and segmenting suspicious contacts.
Engagement metrics: A lack of opens or clicks from a segment of new subscribers after a sudden signup spike is a strong indicator of bot activity.
Deliverability impact: Sending emails to bot-generated addresses can lead to increased bounce rates, spam complaints, and ultimately damage your sender reputation, potentially leading to blocklisting.
Key considerations
Data analysis: Regularly analyze signup rates and engagement metrics to detect unusual spikes or patterns that may indicate bot activity. This proactive monitoring can help you remove bad email addresses from your list more efficiently.
Automated defenses: Implement multi-layered defenses, including CAPTCHA, honeypot fields, and possibly double opt-in, to create a robust barrier against automated sign-ups.
Careful list scrubbing: When cleaning a compromised list, consider the risk of unsubscribes or complaints versus the benefit of maintaining a clean list. It may be better to remove suspicious contacts who show no engagement.
ESP collaboration: Consult with your Email Service Provider (ESP). They often have tools, data, or advice on identifying and mitigating bot activity specific to their platform, as highlighted by Akismet's guide on spambots.
What email marketers say
Email marketers frequently encounter challenges with bot-generated sign-ups, particularly during unusual spikes in subscription rates. Their primary concerns revolve around the difficulty of distinguishing genuine subscribers from bots when the latter use seemingly normal email addresses. The consensus is that while various tools exist for list cleaning, their effectiveness against sophisticated spam traps or bot attacks is limited. Many marketers emphasize the importance of proactive preventative measures on signup forms and the cautious approach to cleaning existing lists, especially when dealing with a high volume of suspicious contacts.
Key opinions
List hygiene priority: Maintaining a clean email list through regular hygiene practices is considered a top priority for marketers to ensure deliverability and engagement.
Engagement campaigns: Re-engagement campaigns are a common strategy to identify inactive subscribers, but they might not be suitable for bot-infested lists due to the risk of complaints.
Preventative measures: Implementing invisible reCAPTCHA and hidden form fields (honeypots) are highly recommended preventative tactics against bot sign-ups, as discussed in best practices for email validation on sign-up.
Data-driven removal: When removing bad sign-ups, it's best to utilize auxiliary data captured during the signup process, such as IP address, referer, or browser identifier, rather than relying solely on the email address itself.
Key considerations
Risk of complaints: Marketers are wary of sending re-engagement emails to potentially bot-generated contacts due to the high risk of spam complaints, which can negatively impact sender reputation and lead to blacklisting.
Accuracy of hidden fields: While effective, marketers should be aware that some browser auto-fill features (e.g., Safari) might unintentionally fill hidden fields, leading to false positives.
Historic data wipe: A drastic but sometimes necessary measure involves identifying the period of bot attack and removing all contacts from that timeframe who have not shown any engagement, a method that can improve welcome series email deliverability.
Form field naming: For honeypots, using obscure field names (e.g., b_b75912d2a4eb0dd37d3ace515_46d88b7e20) makes it less likely for legitimate users' autofill functions to populate them, minimizing false positives, as detailed by WPBeginner in their guide on preventing newsletter signup spam.
Marketer view
Marketer from Email Geeks mentioned concerns about a sudden spike in sign-ups, fearing a bot attack. They expressed worry about potential complaints if they were to send re-engagement emails to these suspicious contacts. Their immediate plan was to implement 'Not a robot' verification for new sign-ups, acknowledging it wouldn't help with existing bot-generated contacts.
26 Nov 2019 - Email Geeks
Marketer view
Marketer from OOPSpam Blog advises protecting your email list from spam bots. They recommend implementing double opt-in and two-factor authentication as essential tactics. Additionally, they suggest utilizing CAPTCHA for signup forms to prevent automated submissions.
15 Jan 2025 - OOPSpam Blog
What the experts say
Email deliverability experts highlight that tools for checking spam email addresses might not always effectively detect sophisticated spam traps or bot-generated addresses that appear legitimate. They emphasize the importance of proactive preventative measures rather than solely relying on post-facto cleanup tools. For existing issues, experts suggest leveraging rich data captured during signup and a strategic approach to list segmentation and removal. Understanding the different types of bot attacks, such as subscription bombing versus SEO spam, helps in tailoring the response effectively.
Key opinions
Data-rich analysis: Removing bad signups is most effectively done by analyzing associated data like IP address, referer, and browser identifiers, as bot-generated emails often appear normal.
Engagement as a filter: After a bot attack, a practical approach is to remove all sign-ups from the affected period that show no subsequent email opens or clicks, which helps in identifying email spam traps.
Bot behavior: SEO spam bots aim to create links to their owners' pages on high-ranking sites. They often interact with signup forms because they can resemble comment submission forms.
ESP consultation: Email Service Providers (ESPs) can offer valuable advice and data specific to the type of bot attack (e.g., subscription bombing vs. SEO spam), helping businesses formulate a targeted response.
Key considerations
Distinguishing bot types: Understanding the different motivations behind bots (e.g., list bombing vs. SEO spam) can help in identifying their signatures and implementing appropriate countermeasures.
Automated vs. human: The preventative measures chosen (e.g., CAPTCHA, honeypots) should be effective against automated bots while minimizing friction for legitimate human users.
Data capture importance: Ensure your signup forms capture sufficient auxiliary data (IP, referrer, user agent) to assist in future identification and mitigation of bot activity. This is vital when considering why spambots submit real emails to signup forms.
Strategic remediation: Don't rush to email suspicious contacts. A measured approach, possibly involving a mass removal of unengaged signups from the affected period, can prevent severe damage to your sender reputation, as advised by SpamResource.
Expert view
Expert from Email Geeks suggested that removing bad signups is best achieved by utilizing supplementary data collected alongside the email address, such as the referer, IP address, or browser identifier. They posited that email addresses themselves are likely to be legitimate, making other data points more crucial for identification.
26 Nov 2019 - Email Geeks
Expert view
Expert from Word to the Wise emphasizes that effective email deliverability depends on a clean list. They stress that sending to invalid or bot-generated addresses can lead to increased bounce rates and spam complaints, signaling poor list management to ISPs and harming sender reputation.
10 Mar 2024 - Word to the Wise
What the documentation says
Official documentation and industry guides frequently recommend a combination of technical measures to combat bot-generated email addresses. These typically include implementing client-side challenges like CAPTCHAs, server-side validation, and integrating 'honeypot' fields into forms. The core principle is to create obstacles that human users can easily bypass but automated bots will fall into, allowing for identification and prevention. Emphasized is the continuous need for list hygiene and understanding various attack vectors, such as list bombing, to protect email deliverability.
Key findings
Honeypot fields: Adding a hidden 'honeypot' field to a form is an effective method to identify list bombing or bot activity. If this field is filled, it indicates bot interaction.
CAPTCHA integration: Utilizing CAPTCHA (e.g., reCAPTCHA) on signup forms helps in distinguishing between human users and bots, reducing automated sign-ups.
Double opt-in: Implementing a double opt-in process ensures that only genuinely interested users confirm their subscription, validating email addresses and preventing false sign-ups.
Email validation services: Many email validation services can help identify and remove fake or invalid email addresses, using various methods to verify their authenticity, which is a key part of protecting signup forms from bots.
Key considerations
Comprehensive approach: A multi-layered defense combining various techniques (e.g., CAPTCHA, honeypot, double opt-in, IP blocking) is more effective than relying on a single method.
Regular list cleaning: Even with preventative measures, ongoing list hygiene, including the removal of unengaged or suspicious contacts, is essential to maintain a healthy list.
User experience: While security is important, measures like CAPTCHA should be implemented carefully to avoid deterring legitimate users and impacting conversion rates.
Domain verification: Verifying the domain of new email addresses can flag suspicious or temporary email domains often used by bots, a strategy covered in strategies for email list validation, also supported by Klaviyo's help center.
Technical article
Documentation from CHEQ recommends using an email validation service to identify and remove fake email addresses. These services utilize various methods to detect invalid or bot-generated entries, helping businesses maintain a clean and effective email list, thereby fighting spam sign-ups and improving deliverability.
20 Sep 2023 - CHEQ
Technical article
Documentation from Klaviyo Help Center suggests adding a 'honeypot' field to a website's form as a straightforward method to identify if a list bombing attack is occurring. If this hidden field is populated, it indicates bot activity, allowing for easy identification and prevention of fake profiles.
