What are reliable methods to identify and prevent bots from submitting forms outside of double opt-in?
Matthew Whittaker
Co-founder & CTO, Suped
Published 23 Jul 2025
Updated 16 Aug 2025
7 min read
Dealing with bot submissions on your website forms can be a constant headache. These automated entries clog your databases with junk data, skew analytics, and can even lead to issues like reduced email deliverability due to increased bounce rates or spam complaints. While double opt-in is a highly effective method for ensuring legitimate sign-ups, it is not always feasible or desired for every type of form, such as contact forms or event registrations.
The good news is that there are many other reliable strategies to identify and prevent bots from submitting forms, even when you cannot implement a double opt-in process. These methods often involve a layered approach, combining technical deterrents with behavioral analysis to create a robust defense system against automated spam.
Detecting bot signatures
Bots operate predictably, often without the nuanced, human-like interactions that real users display. By observing and analyzing these patterns, you can often flag suspicious submissions before they even hit your database. This includes scrutinizing data points like the user-agent string, which identifies the browser and operating system, and the IP address, which reveals geographic location and network type.
A bot's speed of submission is another major indicator. Humans take time to read form fields, type, and click. Bots, however, can fill out and submit forms in milliseconds. Monitoring the time elapsed between the form loading and its submission, sometimes called the "time trap," can effectively identify automated activity. Similarly, analyzing the consistency of mouse movements or click coordinates can reveal bot behavior, as automated scripts tend to click in the exact same spot repeatedly, unlike varied human interaction.
Beyond technical metrics, content analysis of submitted data can also be telling. Spam bots often insert irrelevant keywords, gibberish, or links to malicious sites. Implementing filters that scan for common spam patterns or known suspicious domains can help in this regard. This multi-faceted approach to detection allows for a more comprehensive defense against various types of automated attacks.
Technical defenses against automated submissions
While detection is crucial, proactive measures that prevent bots from submitting forms in the first place are even better. One of the most widely used methods is the honeypot field. This involves adding a hidden field to your form that is invisible to human users but detectable by bots. If this field is filled out upon submission, you know it is a bot and can reject the submission.
Another effective barrier is reCAPTCHA (or other CAPTCHA solutions). While some versions (v2, v3) can introduce slight friction, they are highly effective. reCAPTCHA v3, for instance, operates in the background, assessing user behavior and providing a score without requiring direct interaction, making it more user-friendly. This score can then be used on your backend to determine whether to accept or reject a submission. We have more information about preventing nefarious email signups with reCAPTCHA if you would like to read more about this.
Additionally, many modern forms rely on JavaScript for submission. You can implement a check to ensure that a form was submitted via JavaScript (rather than a direct POST request) by adding a hidden field that is only populated by JavaScript. If the field is empty, it is likely a bot that does not execute JavaScript. This method can be turned on page by page if you notice an increase in bot activity, rather than being on by default, to avoid impacting legitimate users.
Proactive monitoring and list hygiene
Proactive monitoring and ongoing list hygiene are essential to maintain the integrity of your form submissions and, by extension, your email lists. Implementing rate limiting, for example, can restrict the number of submissions allowed from a single IP address within a specific timeframe, effectively preventing bots from flooding your system. If an IP exceeds this threshold, submissions from that address can be temporarily or permanently blocked.
Another strategy involves blocking known spam IPs or even entire geographic regions if your business does not operate there. While a blocklist (or blacklist) may not catch all threats, it is a good baseline defense. Additionally, real-time email validation at the point of entry is critical. Services can check email addresses for common bot patterns, temporary or disposable domains, and syntax errors, filtering out suspicious entries before they even make it to your list. This helps with identifying and preventing spambot traffic.
Best practices for email validation
Syntax validation: Ensure the email address follows standard formatting rules (e.g., user@domain.com).
Disposable email detection: Block registrations from temporary or disposable email address providers.
Role-based email detection: Identify and potentially block addresses like info@, sales@, support@, which are often associated with bots or spam traps.
Domain existence check: Verify that the domain part of the email address actually exists and has valid MX records.
Leveraging data for better protection
Beyond active blocking and validation, a key aspect of preventing bot submissions lies in leveraging data and behavioral analysis. By continuously analyzing the data collected from your forms, you can identify patterns that indicate bot activity. This might include unusually high submission rates during off-peak hours, submissions with identical timestamps, or entries containing suspicious character sequences or email addresses from obscure domains. This is part of a holistic approach to removing bot-generated spam.
Advanced solutions can also analyze user behavior, such as mouse movements, keyboard strokes, and scroll patterns. Human users exhibit natural, varied behavior, while bots often demonstrate predictable, repetitive actions. By comparing user interactions against a baseline of normal human behavior, these systems can flag or block submissions that deviate significantly from typical patterns. This kind of nuanced analysis is crucial as bots become more sophisticated at mimicking human actions.
Combining these data-driven insights with real-time threat intelligence, such as regularly updated blacklists (or blocklists) of known malicious IPs and domains, creates a powerful defense. This layered security approach ensures that even if one detection method is bypassed, others are in place to catch the fraudulent submission. The key is to never rely on a single solution, as bots are constantly evolving to circumvent common protections.
Strengthening your form security
Protecting your forms from bot submissions requires a dynamic and comprehensive approach. By implementing a combination of technical barriers, proactive monitoring, and intelligent data analysis, you can significantly reduce unwanted entries and maintain the quality of your collected data. Remember that bot tactics evolve, so continuous vigilance and adaptation of your defense mechanisms are key to long-term success.
Investing in these strategies ensures that your valuable resources are spent engaging with real, interested individuals, rather than cleaning up after automated intruders. This helps ensure better data quality and improved overall efficiency for your business.
Views from the trenches
Best practices
Implement a multi-layered approach using a combination of techniques, not just one.
Use honeypot fields that are invisible to humans but detectable by bots.
Integrate reCAPTCHA v3 for frictionless bot detection based on user behavior scoring.
Utilize real-time email validation to filter out disposable and role-based email addresses.
Analyze user-agent strings and IP addresses for suspicious patterns.
Common pitfalls
Relying solely on one bot detection method, as bots can easily bypass single defenses.
Overly aggressive blocking that legitimate users might find frustrating, leading to false positives.
Not regularly updating or adapting your anti-bot measures, allowing new bot techniques to succeed.
Failing to analyze email content or domain validity, which can allow sophisticated spam through.
Ignoring the impact of bot sign-ups on email deliverability and sender reputation.
Expert tips
Consider a server-side check for JavaScript submissions by adding a hidden field dynamically, only activated if JS is enabled.
Implement rate limiting per IP address to prevent a single source from flooding your forms.
Track mouse movement and click coordinates, as bots often exhibit consistent, non-human patterns.
Regularly review your form submission data for unusual trends or new bot signatures.
Geoblock IP ranges from high-risk countries if your target audience is geographically limited.
Marketer view
Marketer from Email Geeks says there is no single way to detect bots, and AWeber uses dozens of techniques and billions of data points to identify and filter out bots attempting to fill out user forms.
2022-02-20 - Email Geeks
Marketer view
Marketer from Email Geeks says that a layered approach is essential, starting at the network level and extending to post-subscription processes.
