How can I identify and prevent spam/bot traffic at email subscription points?

Summary

Combating spam and bot traffic at email subscription points requires a layered approach. Standard practices like CAPTCHA and double opt-in are foundational, but increasingly sophisticated bots necessitate advanced techniques. These include capturing signup audit trails, using tiered CAPTCHAs, implementing honeypot fields (hidden forms), performing reverse DNS lookups, and analyzing signup behavior for suspicious patterns. Server-side validation (using PHP) is crucial as bots often bypass client-side JavaScript. Rate limiting, bot management tools (like Cloudflare), and blocking known spam IPs/countries provide additional layers of defense. Integrating with services like Project HoneyPot and maintaining strong transactional relationships are also beneficial. However, the effectiveness of CAPTCHA is diminishing due to captcha farms, and reliance on ESPs for subscription checks is essential, requiring proactive communication with their abuse desks.

Key findings

  • Layered Security: A multi-layered security approach combining standard practices with advanced techniques is essential.
  • Server-Side Validation: Server-side validation is crucial to prevent bots from bypassing client-side JavaScript.
  • Behavioral Analysis: Analyzing signup behavior (location, time, referrer) helps identify suspicious patterns.
  • Honeypot Effectiveness: Honeypot fields (hidden form fields) can trap bots without affecting legitimate users.
  • Third-Party Tools: Tools like Google reCAPTCHA, Cloudflare's bot management, and Project HoneyPot offer advanced bot detection capabilities.
  • Proactive ESP Engagement: Engaging with the ESP's abuse desk is critical for proactive defense against subscription bombing.
  • DNS Lookups: Performing reverse DNS lookups helps to verify the sources.

Key considerations

  • CAPTCHA Limitations: CAPTCHA's effectiveness is declining due to the rise of captcha farms and advanced bot techniques.
  • ESP Dependence: The success of subscription checks relies heavily on the ESP's capabilities and responsiveness.
  • False Positives: Advanced risk analysis systems may occasionally flag legitimate users as bots, requiring careful monitoring and adjustments.
  • Maintenance: The bot landscape is ever changing, requiring regular maintenance.

What email marketers say
14Marketer opinions

To combat spam and bot traffic at email subscription points, marketers employ various techniques. Standard practices include CAPTCHA and double opt-in, with some suggesting hidden form fields (honeypots) in the HTML to deter automated sign-ups. Analyzing signup behavior, such as location, time, and referrer, can flag suspicious activity. While Mailchimp supports hidden fields, some advocate for server-side PHP solutions to validate forms. Restricting role-based accounts (abuse@, postmaster@) is also recommended. CAPTCHA's effectiveness is questioned due to captcha farms. Utilizing IP address blocking, JavaScript validation, analyzing form completion time, and blocking specific countries add further layers of defense.

Key opinions

  • Standard Practices: CAPTCHA and double opt-in are commonly used to verify human subscribers.
  • Honeypot Fields: Hidden form fields can effectively trap bots without impacting user experience.
  • Behavioral Analysis: Analyzing signup patterns (location, time, referrer) can identify suspicious behavior indicative of bots.
  • Server-Side Validation: Using PHP for form validation on the server-side ensures bots cannot bypass checks.
  • Account Restrictions: Blocking role-based email addresses (abuse@, postmaster@) prevents abuse.
  • Analytics: Analysing signup behaviour can flag suspicious behaviours for manual inspection.
  • Country blocking: Blocking countries that are not likely to be the right target audience

Key considerations

  • CAPTCHA Limitations: Captcha farms can circumvent CAPTCHA, diminishing its effectiveness.
  • Hidden Field Effectiveness: Hidden fields are not as effective.
  • PHP Implementation: PHP on the server-side is best as javascript validation is not always effective.
Marketer view

Marketer from Email Geeks suggests that depending on scale, one can run analytics based on typical sign-up behaviour to flag suspicious behaviours for manual inspection, using data-points like location, time, referrer, etc.

September 2022 - Email Geeks
Marketer view

Email marketer from Reddit suggests analysing the time it takes a user to fill out the form, and identify if it is quicker than a human could reasonably do so.

December 2022 - Reddit
Marketer view

Email marketer from HubSpot shares that blocking known spam IP addresses from accessing signup forms can help reduce the amount of bot traffic and prevent fake subscriptions.

March 2021 - HubSpot
Marketer view

Email marketer from LinkedIn suggests blocking signups from countries that aren't likely to be the right target audience.

November 2021 - LinkedIn
Marketer view

Email marketer from Reddit shares that using honeypot traps, which are hidden form fields only bots will fill out, can help identify and block bot submissions without affecting the user experience for human subscribers.

July 2021 - Reddit
Marketer view

Marketer from Email Geeks mentions that the hidden form field is not as effective as it was and warns against relying on RECAPTCHA (even v3) because of the growth of captcha farms (people hired to do the work of bots but are actually humans). A good tool but, like everything, far from perfect.

January 2022 - Email Geeks
Marketer view

Email marketer from Mailchimp shares that enabling signup form security features, such as CAPTCHA and double opt-in, can help protect against spam and bot traffic, ensuring only genuine subscribers are added to the list.

July 2024 - Mailchimp
Marketer view

Marketer from Email Geeks states that Mailchimp supports hidden fields and that it is not an "enterprise feature" by any means.

February 2025 - Email Geeks
Marketer view

Email marketer from StackExchange shares that implement JavaScript validation to only allow emails with valid formats through and also look for common disposable email address providers.

October 2022 - StackExchange
Marketer view

Marketer from Email Geeks suggests certain types of role and other accounts should never be subscribed, such as abuse@, postmaster@, support@, and others, and ideally, they should be caught at the form level.

October 2024 - Email Geeks
Marketer view

Email marketer from Neil Patel explains that implementing CAPTCHA on signup forms can help differentiate between human users and bots, preventing automated spam submissions.

December 2021 - Neil Patel
Marketer view

Marketer from Email Geeks shares that CAPTCHA and confirmed opt-in are standard practices and suggests using a hidden form in the HTML for the signup to deter automated sign-ups.

January 2025 - Email Geeks
Marketer view

Email marketer from Sendinblue shares that using double opt-in requires subscribers to confirm their email address before being added to the list, verifying the user's intention and preventing bots from subscribing with fake or harvested emails.

July 2024 - Sendinblue
Marketer view

Marketer from Email Geeks suggests that one piece of PHP could serve the form (with the hidden value), then check it, and only pass on the subscribes that didn't have it filled to the ESP API.

May 2021 - Email Geeks

What the experts say
7Expert opinions

Experts recommend a multi-faceted approach to identifying and preventing spam and bot traffic during email subscription. Capturing a detailed audit trail (timestamp, IP, browser data), employing tiered CAPTCHAs, and performing reverse DNS lookups are crucial. Hidden form fields act as traps, particularly with JavaScript. Server-side ESP checks are necessary as many bots bypass client-side validation. A strong transactional relationship is important. Consulting with the ESP's abuse desk is advised before involving the development team.

Key opinions

  • Audit Trails: Detailed signup audit trails (timestamp, IP, browser data) aid in identifying suspicious signups.
  • Tiered CAPTCHAs: Tiered CAPTCHAs balance security with user experience by presenting increasingly complex challenges.
  • Reverse DNS Lookups: Reverse DNS lookups help verify the legitimacy of subscriber IP addresses.
  • Hidden Form Fields: Hidden form fields can deter bots
  • ESP Checks: Server-side checks performed by the ESP are critical.
  • Transactional Relationship: A strong transactional relationship helps.

Key considerations

  • ESP Dependence: The effectiveness relies on the ESP's capabilities to handle subscription checks and abuse reports.
  • Development Involvement: Consulting the ESP's abuse desk is crucial before involving the development team.
Expert view

Expert from SpamResource suggests performing reverse DNS lookups on the IP addresses of subscribers. If the IP address doesn't have a valid reverse DNS record, or if it resolves to a dynamic hostname, it could be a sign of a bot or spammer.

October 2024 - SpamResource
Expert view

Expert from Email Geeks states that ESPs that can't handle subscription checks are a problem, and that subscription checks need to be server side, as most dumber bots aren't handling JS.

August 2023 - Email Geeks
Expert view

Expert from Word to the Wise mentions that it is important to have a strong transactional realtionship. As this will help filter out bots.

May 2022 - Word to the Wise
Expert view

Expert from Email Geeks recommends talking to the ESPs abuse desk before the dev team and asking them what they're doing about subscription bombing and how one can help.

March 2022 - Email Geeks
Expert view

Expert from SpamResource explains that one approach is to use a tiered CAPTCHA. Start with a simple CAPTCHA, and if the user fails, present a more complex one. This helps weed out simpler bots while providing a smoother experience for legitimate users.

March 2025 - SpamResource
Expert view

Expert from Email Geeks suggests capturing an audit trail for each signup, including timestamp, remote IP, and browser metadata. Also suggests considering zerocaptcha and fraud tracking blacklists. Adding a hidden form in the HTML, especially with Javascript, will deter many bots.

May 2022 - Email Geeks
Expert view

Expert from Email Geeks says that the easiest thing to add is a hidden form field with a value in it; if the POST doesn't have that value, discard it. More subtle approaches include actual text fields hidden by CSS, and discarding the submission if the contents change.

March 2024 - Email Geeks

What the documentation says
4Technical articles

Technical documentation emphasizes using advanced tools and techniques to identify and prevent bot traffic at email subscription points. Google reCAPTCHA employs risk analysis, OWASP recommends rate limiting, Cloudflare advocates bot management tools, and Project HoneyPot offers a distributed system for identifying malicious networks.

Key findings

  • Risk Analysis: Google reCAPTCHA uses advanced risk analysis to differentiate between humans and bots.
  • Rate Limiting: OWASP suggests rate limiting to restrict the number of requests from a single IP address.
  • Bot Management Tools: Cloudflare recommends using bot management tools to analyze traffic patterns.
  • Network Identification: Project HoneyPot provides a system for identifying malicious bot networks.

Key considerations

  • Integration Complexity: Implementing these solutions may require technical expertise and integration with existing systems.
  • False Positives: Advanced risk analysis and bot management tools may occasionally flag legitimate users as bots.
  • Maintenance: These systems require ongoing maintenance and updates to remain effective against evolving bot techniques.
Technical article

Documentation from Project HoneyPot provides a distributed system for identifying malicious bot networks, you can use a tool like this to identify networks of bots signing up.

March 2024 - Project HoneyPot
Technical article

Documentation from OWASP explains that implementing rate limiting on signup forms restricts the number of requests from a single IP address within a specific timeframe, preventing bots from flooding the system with spam subscriptions.

September 2024 - OWASP
Technical article

Documentation from Google reCAPTCHA explains that reCAPTCHA uses advanced risk analysis techniques to distinguish between humans and bots, offering various challenges or background analysis to prevent automated abuse on signup forms.

November 2023 - Google reCAPTCHA
Technical article

Documentation from Cloudflare explains that using bot management tools, which analyze traffic patterns and identify malicious bots, can help prevent automated spam subscriptions and protect signup forms from abuse.

October 2022 - Cloudflare