Why do spambots submit real emails to signup forms?
Matthew Whittaker
Co-founder & CTO, Suped
Published 1 May 2025
Updated 19 Aug 2025
7 min read
Dealing with an influx of unwanted sign-ups on your website forms, especially when they use real email addresses, can be incredibly frustrating. I've seen this issue repeatedly, and it raises a crucial question: What do spambots gain by submitting legitimate email addresses with fake or irrelevant information?
The answer isn't always straightforward, but it boils down to several malicious objectives, ranging from simple digital nuisance to complex cyber-attacks. Understanding these motives is the first step in building a robust defense for your email lists and maintaining your sender reputation.
The motivations behind the attacks
When spambots submit real email addresses to your forms, it's rarely random. There are usually calculated reasons behind such actions, which can severely impact legitimate email users and the businesses whose forms are exploited. This behavior often falls into two primary categories of malicious intent.
Subscription bombing and harassment
One significant reason is subscription bombing. In this scenario, criminals use automated scripts to sign up a target's email address to hundreds or even thousands of mailing lists, newsletters, and services. The intent is to deluge the victim's inbox with an overwhelming volume of legitimate (though unwanted) confirmation emails and newsletters. This tactic is often employed to achieve one of two goals:
Hide criminal activity: The deluge of emails can serve as a smoke screen to hide notifications about fraudulent transactions or account takeovers. For example, if a criminal uses a stolen credit card to make a purchase, the victim might receive an email confirmation. By burying this critical email under hundreds of spammy subscriptions, the criminal hopes the victim won't notice the fraudulent activity until it's too late.
Harassment: It can also be a simple act of digital harassment, overwhelming someone's inbox to cause frustration and disruption. This is a common form of abuse on the internet.
These attacks are not about getting their messages into an inbox, but about flooding someone else's with unwanted noise.
SEO spam and exploiting forms
Another reason stems from SEO spam bots. These bots often crawl the web looking for any kind of input field to inject spam content, typically with links back to their clients' websites. Their programming can be quite rudimentary, meaning they don't distinguish between a comment form, a contact form, or a newsletter signup form. If it looks like a field they can type into and submit, they'll try.
While they might intend to leave a comment with a link, they end up submitting data to your signup form. Sometimes, they'll use real email addresses from scraped lists, not necessarily because those specific emails are targets, but because having a seemingly valid email address helps their submission appear more legitimate to basic form validation. This can lead to your system sending confirmation emails or welcome messages to uninterested parties, causing confusion and potentially increasing spam complaints.
Impact on email deliverability
Whether it's subscription bombing or accidental SEO spam, the outcome for your email program is often negative. These bot sign-ups can significantly harm your email deliverability and overall sender reputation, regardless of whether the submitted emails are fake or real.
Negative impact on sender reputation
When your email service provider (ESP) sends emails to addresses that didn't genuinely opt-in, it can trigger a cascade of issues. If these real but unsolicited emails start generating high unsubscribe rates or, worse, spam complaints, mailbox providers like Gmail and Yahoo take notice. They interpret these signals as a sign that your sending practices are poor, potentially leading to your emails being directed to the spam folder, or even having your IP or domain placed on a blacklist (or blocklist). This is a direct hit to your sender reputation, making it harder for your legitimate emails to reach the inbox.
Furthermore, a high volume of sign-ups from bots, even with real email addresses, can skew your engagement metrics. It becomes difficult to accurately assess the performance of your campaigns when your lists are bloated with uninterested or non-existent subscribers. This can lead to misinformed marketing decisions and wasted resources.
Impact of fake email signups
Hard bounces: Emails sent to non-existent addresses result in hard bounces, which severely damage your sender reputation and can lead to being put on a blacklist.
Wasted resources: Your ESP charges are often based on list size or emails sent, meaning you pay for sending to invalid addresses.
Skewed metrics: Open and click rates appear lower, distorting your campaign performance data.
Impact of real unsolicited email signups
Spam complaints: Recipients who never signed up are likely to mark your emails as spam, leading to significant reputation damage.
High unsubscribes: Even if they don't complain, a high volume of unsubscribes signals disinterest to mailbox providers.
List hygiene issues: Your legitimate audience becomes harder to identify amidst the noise, making segmentation and personalization challenging. This problem is further explored in how to prevent bot sign-ups.
Identifying and preventing bot attacks
Protecting your email signup forms from spambots is crucial for maintaining a healthy email program and strong sender reputation. It involves a combination of technical measures and diligent monitoring.
Implement robust anti-bot measures
There are several strategies to employ to prevent bot sign-ups. One effective method is to use double opt-in, which requires users to confirm their subscription via an email link. This ensures that only legitimate email addresses with active users are added to your list, effectively stopping subscription bombing in its tracks. You can read more about this in how to protect email list forms from bots.
Another powerful technique is the honeypot method. This involves adding a hidden field to your form that is invisible to human users but detectable by bots. If a bot fills out this field, you know it's not a legitimate submission and can block it. It's a simple yet highly effective way to filter out automated spam.
For more advanced bot detection, consider implementing CAPTCHAs or similar challenges. While sometimes criticized for user experience, they remain a strong barrier against automated submissions. Services like Mailchimp and others actively combat fake sign-ups, as noted in their help documentation, by suggesting such preventative methods. It is also important to maintain regular blocklist monitoring to quickly detect and address any reputation issues.
Monitoring and list hygiene
Beyond technical measures, regularly monitoring your subscriber list for suspicious activity is key. Look for unusual spikes in sign-ups, particularly from generic domains or unusual naming patterns. Quick detection allows you to purge these unwanted entries before they cause lasting damage.
Detecting suspicious sign-ups
Identifying bot activity often requires vigilance and analysis of your subscription data. Here are common indicators:
High volume: Sudden, unexplained spikes in sign-ups over a short period.
Unusual patterns: Sequential or random characters in names, or specific common generic email addresses. You can learn more about potential reasons for spam addresses.
Geographic anomalies: Sign-ups from unexpected countries or regions.
Referral sources: Sign-ups from suspicious or unknown referral domains.
Regularly reviewing your new sign-ups and segmenting them can help identify these trends. If you find a block of suspicious sign-ups, it's best to remove them from your list immediately and investigate the source of the bot activity. This proactive approach helps protect your sender reputation and ensures your emails reach genuine subscribers. Learn more about identifying and removing bot-generated spam emails from your lists.
Views from the trenches
Best practices
Implement double opt-in on all your email subscription forms to confirm genuine interest and ownership of the email address.
Utilize honeypot fields in your forms, which are invisible to human users but capture bot submissions, allowing you to filter them out.
Regularly monitor your new sign-up metrics for unusual spikes or patterns, such as a sudden increase in subscriptions from unusual locations or generic domains.
Use CAPTCHAs or reCAPTCHAs for sensitive forms, balancing security with user experience to deter automated entries effectively.
Common pitfalls
Relying solely on single opt-in exposes your list to fake sign-ups and subscription bombing, which can quickly harm your sender reputation.
Ignoring suspicious sign-up patterns can lead to inflated lists with poor engagement, resulting in higher bounce rates and spam complaints.
Failing to regularly clean your email list by removing unengaged or invalid subscribers allows bot-generated entries to accumulate.
Using overly complex CAPTCHAs that frustrate legitimate users, potentially leading to lost sign-ups and a poor user experience.
Expert tips
For transactional forms, consider rate limiting submissions from individual IP addresses to prevent rapid, automated attacks.
Integrate an email validation API at the point of sign-up to instantly check the validity and deliverability of submitted email addresses.
Analyze server logs and traffic patterns to identify bot-like behavior, such as unusually fast form submissions or repeated access from the same IP.
Beyond technical solutions, educate your team on the potential impacts of form spam and the importance of maintaining list hygiene.
Marketer view
A marketer from Email Geeks says they assume spambots cause issues for personal gain, enjoying the disruption they create for innocent people and businesses.
2019-04-12 - Email Geeks
Expert view
An expert from Email Geeks says there are two main causes: SEO spam bots that mistake signup forms for comment sections and subscription bombing used by criminals.
2019-04-12 - Email Geeks
Protecting your email ecosystem
Spambots submitting real emails to signup forms are not just a nuisance, they pose a genuine threat to your email deliverability and overall marketing efforts. The motivations range from disrupting legitimate email accounts to exploiting your forms for SEO spam or even more sinister activities like hiding financial fraud.
By understanding these underlying reasons, you can implement effective countermeasures. Combining robust technical solutions like double opt-in and honeypots with continuous monitoring of your subscriber list is essential. Protecting your forms means protecting your sender reputation, ensuring your legitimate messages reach their intended audience, and maintaining the integrity of your marketing data.
Gmail and 
Yahoo take notice. They interpret these signals as a sign that your sending practices are poor, potentially leading to your emails being directed to the spam folder, or even having your IP or domain placed on a blacklist (or blocklist). This is a direct hit to your sender reputation, making it harder for your legitimate emails to reach the inbox.
