How to prevent bot sign-ups and suspicious contacts on email lists?
Matthew Whittaker
Co-founder & CTO, Suped
Published 13 Aug 2025
Updated 14 Aug 2025
7 min read
Summary
Bot sign-ups and suspicious contacts can severely degrade the quality of an email list, leading to reduced deliverability and potential blocklist (or blacklist) listings. These fraudulent sign-ups often originate from webform hijacking attempts, where automated bots inject fake or compromised email addresses into your database. Identifying and addressing these issues is crucial for maintaining a healthy sender reputation and ensuring your legitimate emails reach the inbox.
Key findings
Impact on deliverability: Spam bots populate lists with invalid or compromised email addresses, which can lead to high bounce rates and trigger spam traps, negatively impacting sender reputation and email deliverability.
Common vectors: Webform hijacking is a primary method for bots to infiltrate email lists, affecting various opt-in points across a website.
Preventive measures: Implementing CAPTCHA (especially invisible reCAPTCHA), double opt-in, honeypot fields, and real-time email validation are effective strategies.
CDN benefits: Placing websites behind a Content Delivery Network (CDN) like Cloudflare can help filter out malicious traffic before it reaches forms.
Data breaches: Bots may use email addresses acquired from dark web data dumps, often attempting to bury legitimate notifications in compromised inboxes.
Key considerations
Comprehensive protection: Apply bot prevention measures across all opt-in points, including newsletter sign-ups, contact forms, and even less obvious areas like wishlist features in e-commerce platforms (e.g., Magento).
Proactive cleaning: Regularly review and clean your email lists to remove suspicious or invalid contacts, as continued mailing to these can lead to spam trap hits and blocklist listings.
Layered security: Combine multiple defensive techniques, such as CAPTCHA, honeypots, and proper email validation, to create a robust defense against evolving bot tactics.
External resources: Refer to guides on protecting email lists from bots for more detailed strategies.
What email marketers say
Email marketers frequently face the challenge of bot-generated sign-ups, which can dilute audience quality and lead to deliverability issues. Their experiences highlight practical strategies for defending against these unwanted contacts, often emphasizing the immediate benefits of certain security implementations.
Key opinions
Form protection: Marketers frequently point to webform hijacking as the primary cause of suspicious sign-ups, underscoring the need for immediate security on all public-facing forms.
CAPTCHA effectiveness: Many marketers find CAPTCHA, particularly invisible implementations like Google's reCAPTCHA, to be highly effective at blocking bots without hindering user experience.
Double opt-in vs. CAPTCHA: While double opt-in adds a layer of verification, some marketers prefer CAPTCHA as a first line of defense to avoid sending emails to bot-generated, potentially invalid, addresses.
Bot motivation: Bots often subscribe with compromised email addresses to flood inboxes, burying legitimate alerts related to account breaches or suspicious activity.
E-commerce specific attacks: Features like 'share wishlist' in e-commerce platforms (e.g., Magento) are recognized as common attack vectors for bot sign-ups.
Key considerations
Universal implementation: Security measures should be integrated across all site entry points, including overlays, embedded forms, and checkout processes, to ensure comprehensive protection against bots.
CDN integration: Consider using a CDN like Cloudflare to add an additional layer of security, filtering malicious traffic before it impacts your website's forms.
Proactive list cleansing: Regularly audit your lists for suspicious entries and promptly remove them to mitigate potential deliverability issues.
Stay informed: Be aware of emerging threats, such as new email addresses appearing on the dark web, which could signal future bot attacks.
Further reading: Explore advice on preventing newsletter spam sign-ups from other marketing professionals.
Marketer view
Marketer from Email Geeks observes that contacts with unusual Russian URLs making it onto an email list are likely the result of a webform hijacking attempt.
22 Feb 2019 - Email Geeks
Marketer view
Marketer from Email Geeks strongly recommends ensuring that all webforms on a client's site are adequately protected by CAPTCHA to prevent bot infiltration.
22 Feb 2019 - Email Geeks
What the experts say
Deliverability experts offer strategic and technical perspectives on combating bot sign-ups, emphasizing the long-term health of an email program. Their advice extends beyond simple form protection, delving into systemic vulnerabilities and advanced defensive tactics.
Key opinions
Multi-layered defense: Experts consistently advise that a single protection method is insufficient against sophisticated bots; a combination of technical measures is necessary.
Proactive vs. reactive: Preventing bot entries at the source is far more effective and less damaging to sender reputation than trying to clean lists after they've been compromised.
Behavioral analysis: Advanced solutions involve analyzing user behavior during sign-up to differentiate between legitimate human users and automated scripts.
Vulnerability assessment: Bot activity often signals broader security vulnerabilities within a website or platform that need addressing.
Software hygiene: Keeping all website software, plugins, and platforms updated is critical to patching security gaps exploited by bots.
Key considerations
Regular audits: Conduct periodic reviews of your sign-up flows and data to identify any unusual patterns or suspicious activity that could indicate bot infiltration.
WAF implementation: Consider deploying a robust Web Application Firewall (WAF) to provide an additional layer of defense against malicious automated traffic.
Email service provider collaboration: Understand the built-in defenses offered by your Email Service Provider (ESP) and ensure your website's protection complements them effectively. For general diagnostics, utilize an email deliverability test.
Continuous monitoring: The threat landscape evolves, so ongoing monitoring and adaptation of security measures are essential.
Expert insights: Refer to expert analyses on email abuse and deliverability for deeper understanding.
Expert view
Deliverability expert from Email Geeks advises that proactive measures are crucial for protecting email lists from automated sign-ups, which can severely impact sender reputation.
22 Feb 2019 - Email Geeks
Expert view
Deliverability expert from Email Geeks suggests that a combination of security layers, rather than a single solution, offers the most robust defense against persistent bot activity.
22 Feb 2019 - Email Geeks
What the documentation says
Official documentation from various security and email platforms provides definitive guidelines and best practices for preventing automated sign-ups and maintaining data integrity. These resources often detail the technical implementations and reasoning behind recommended security measures.
Key findings
CAPTCHA validation: Many documentation sources emphasize CAPTCHA as a foundational tool for distinguishing between human users and automated bots during sign-up.
Double opt-in benefits: Double opt-in is frequently recommended as an essential tactic to confirm subscriber intent and protect email lists from spam bots.
Honeypot fields: Technical documentation often describes honeypot fields as an effective, invisible trap for bots, allowing human users to bypass them seamlessly.
Real-time bot detection: Solutions that offer real-time bot detection are highlighted for their ability to prevent spam sign-ups as they occur.
Platform-specific tools: Email marketing platforms often include built-in features, such as CAPTCHA, to assist users in protecting their sign-up forms.
Key considerations
Configure verification tools: Properly set up and integrate verification tools like reCAPTCHA into all your web forms and sign-up processes.
Regular updates: Keep all website platforms, plugins, and security tools updated to ensure you have the latest defenses against new bot tactics.
Data cleaning: Follow guidance on how to identify and remove fake email addresses from your audience to prevent deliverability issues.
Resource consultation: Consult official guides on preventing spam sign-ups for detailed implementation steps and best practices.
Technical article
Documentation from CHEQ explains that deploying a real-time bot detection solution is a highly effective method for preventing spam sign-ups and maintaining clean data.
22 Mar 2025 - CHEQ
Technical article
Documentation from OOPSpam outlines that implementing a double opt-in process is a foundational tactic to protect email lists from being contaminated by spam bots and fake registrations.
