How do bots get on email lists and what kind of addresses do they use?

Bots commonly use disposable email addresses and randomly generated usernames to sign up for lists. They often target forms without adequate CAPTCHA or honeypot protection, sometimes as part of a list bombing attack to overwhelm inboxes and hide malicious activity related to the real email owner's compromised accounts.

What impact do bot sign-ups have on email deliverability?

Bot sign-ups inflate your list with invalid contacts, leading to high bounce rates and spam complaints. These negative signals can severely damage your sender reputation with ISPs and mailbox providers, causing your legitimate emails to land in spam folders or be outright rejected. It also skews your analytics and wastes sending resources.

What are the most effective methods to prevent bot sign-ups?

Implement a combination of defenses: reCAPTCHA or hCAPTCHA on all forms, honeypot fields, double opt-in, email verification at sign-up, and IP rate limiting. Consider using a CDN for additional bot protection.

How can I identify if my list has suspicious contacts or bot sign-ups?

Regularly monitor your bounce and spam complaint rates. Look for unusual spikes in sign-ups from suspicious IP addresses or domains. Utilize email verification services to clean your list, and periodically check your blocklist status . Pay attention to DMARC reports for insights into authentication failures.

Is double opt-in necessary, or can I use single opt-in?

While single opt-in offers a smoother user experience, it makes you more vulnerable to bot sign-ups and invalid addresses. Double opt-in is highly recommended because it requires subscribers to confirm their intent, virtually eliminating bot-generated and mistyped email addresses, leading to a much cleaner and more engaged list.

How to prevent bot sign-ups and suspicious contacts on email lists? - Sender reputation - Email deliverability - Knowledge base

Bot sign-ups and suspicious contacts are a persistent problem for email marketers and website owners. These automated registrations can inflate your list size with invalid addresses, leading to significant deliverability issues and reputation damage. When your email list is filled with bots, your bounce rates soar, engagement metrics plummet, and legitimate emails may end up in spam folders.

Beyond deliverability, there's a more malicious side: list bombing (or email bombing). This occurs when bots mass-subscribe an email address to hundreds or thousands of newsletters, often as a distraction technique to hide security breaches or other fraudulent activities on a user's account. This can overwhelm an individual's inbox, making it difficult for them to spot critical alerts.

Identifying and preventing these unwanted sign-ups is crucial for maintaining a healthy email list and protecting your sender reputation. A clean, engaged list ensures your messages reach actual human recipients, improving your campaign performance and overall email program effectiveness.

Understanding bot sign-ups and their impact

Bot sign-ups typically originate from automated scripts that crawl the web, looking for forms to submit. These scripts don't care about the content or purpose of your list, they just want to fill it with data, often using randomly generated or disposable email addresses. Sometimes, these bots are used in sophisticated attacks, like web form hijacking, where they exploit vulnerabilities to inject malicious content or overload systems.

The primary impact of these fake sign-ups is on your email deliverability. Internet Service Providers (ISPs) and mailbox providers like

Gmail and

Yahoo monitor metrics like bounce rates, spam complaints, and engagement. A high volume of invalid addresses or unengaged subscribers signals poor list hygiene, which can severely damage your sender reputation. This can lead to your legitimate emails being blocked or routed to the spam folder, even for real subscribers. In severe cases, your sending IP or domain could end up on an email blacklist or blocklist, completely halting your email campaigns.

Another concern is the potential for these bots to trigger spam traps. Spam traps are email addresses specifically designed to catch spammers. If a bot signs up an address that is a spam trap, or if you acquire an old list containing recycled spam traps, sending emails to them can immediately flag you as a spammer, leading to your domain being put on a blocklist. You can learn more about spam traps and how they work in our comprehensive guide.

Implementing robust security measures on your sign-up forms is the first and most critical step in preventing bot sign-ups. The goal is to distinguish between human users and automated scripts without creating excessive friction for legitimate subscribers.

One of the most effective methods is CAPTCHA or reCAPTCHA. These challenges, whether image-based puzzles or invisible background checks, are designed to be easy for humans but difficult for bots. For instance, Google's reCAPTCHA v3 operates in the background, assessing user behavior without requiring direct interaction, making the user experience seamless while still providing strong bot protection. Another option is a honeypot field. This is a hidden field in your form that human users won't see or fill out, but bots will. If the field is populated, you know it's a bot submission and can discard it without further processing.

Email verification at the point of sign-up can also be highly effective. This involves validating the email address format and checking against known databases of disposable or suspicious domains. Some services even offer real-time verification to ensure the email address is legitimate and active before it's added to your list. For more detailed insights into protecting your forms, you can read our guide on how to prevent spam bot signups.

It is important to implement these protections across all your opt-in sources, including embedded forms, pop-ups, and checkout processes. While checkout forms may see slightly less bot activity due to the payment barrier, they are not immune.

CAPTCHA vs. Double Opt-in

CAPTCHA (e.g., Cloudflare Turnstile): Verifies human interaction at the point of submission, preventing fake sign-ups immediately.
Honeypot fields: Hidden form fields that only bots will fill, allowing you to catch and block automated submissions.
Disposable email blocking: Prevent sign-ups from temporary email addresses commonly used by bots.

Advanced strategies for list hygiene and defense

While immediate form protections are vital, several other strategies can help you manage and prevent suspicious contacts. Double opt-in is a powerful tool, requiring new subscribers to confirm their email address by clicking a link in a confirmation email. This ensures that only genuinely interested users (and not bots) are added to your list, significantly reducing invalid addresses and potential spam complaints. We have a detailed guide on preventing nefarious email signups that covers these methods.

Another layer of defense involves IP blocking. If you identify a specific IP address or range that is consistently generating bot sign-ups, you can block those IPs from accessing your forms or even your website. Implementing a Content Delivery Network (CDN) like

Cloudflare can also help, as many CDNs offer built-in bot protection and WAF (Web Application Firewall) features that filter malicious traffic before it reaches your server. These services can protect your forms at the DNS level.

Additionally, consider implementing rate limiting on your forms to prevent rapid, automated submissions from a single IP address. This limits how many submissions can occur within a certain timeframe, effectively slowing down bot attacks. Regular monitoring of your form submission data for anomalies, such as high volumes from unusual geographic locations or suspicious email domain patterns (e.g., a surge in .ru addresses or gibberish domains), is also key.

Monitoring and proactive measures

Even with preventative measures, some suspicious contacts might still slip through. Regular monitoring and proactive list management are essential to maintain a healthy email list and strong sender reputation.

Monitor your email performance metrics closely, especially bounce rates and spam complaint rates. An unexplained spike in either of these could indicate an influx of bot sign-ups. Utilize tools that provide detailed reporting, such as Google Postmaster Tools, to gain insights into your sending reputation and potential issues. Pay attention to DMARC reports, which can provide valuable data on email authentication failures and help you identify potential abuse of your domain. Our DMARC monitoring service can help you interpret these complex reports.

Regularly clean your email list to remove unengaged subscribers and invalid addresses. This process helps maintain a high-quality list, which in turn boosts your email deliverability. Consider using email verification services that can identify and remove bot-generated or disposable email addresses. Also, make sure you're monitoring for any listings on an email blacklist (or blocklist) using a blocklist checker. Being listed on a blocklist can severely impact your ability to reach inboxes. If you find yourself on a blocklist, initiate delisting requests promptly.

Finally, stay informed about the latest bot threats and security best practices. The landscape of email security is constantly evolving, and staying proactive is your best defense against sophisticated bot attacks and list bombing attempts. For more on this, explore our comprehensive guide on spam traps.

Views from the trenches

Best practices

Use a multi-layered approach to form protection, combining CAPTCHA and honeypot fields.

Implement double opt-in for all new subscribers to confirm their email address validity.

Regularly monitor your email list for suspicious activity and unusual sign-up patterns.

Leverage email verification services to pre-emptively identify and remove invalid contacts.

Common pitfalls

Relying solely on one security measure, as bots can often bypass single protections.

Neglecting to clean your email list, which leads to accumulated invalid addresses and spam traps.

Ignoring sudden increases in bounce rates or spam complaints, indicating bot activity.

Not protecting all sign-up points on your website, leaving vulnerabilities open for bots.

Expert tips

Consider blocking sign-ups from known disposable email domains to prevent transient bot registrations.

Implement IP rate limiting on your forms to mitigate automated, rapid submission attempts.

Utilize a CDN with WAF capabilities to filter malicious traffic before it reaches your forms.

Regularly review DMARC reports for insights into email authentication failures and potential abuse.

Marketer view

A marketer from Email Geeks says that unusual URLs in profile names on email lists can indicate a webform hijacking attempt.

2019-02-22 - Email Geeks

Marketer view

A marketer from Email Geeks says that ensuring all web forms are protected by CAPTCHA can help prevent bot sign-ups. They note that CAPTCHA is generally more effective than double opt-in for this purpose.

2019-02-22 - Email Geeks

Protecting your list for optimal email deliverability

Preventing bot sign-ups and managing suspicious contacts on your email lists is a continuous effort that combines technical defenses with proactive list hygiene. By implementing multi-layered form protection, leveraging confirmation processes like double opt-in, and regularly monitoring your list health, you can significantly reduce the impact of automated threats.

A clean email list is not just about avoiding deliverability problems, it's about maximizing the effectiveness of your email marketing efforts. When your messages reach engaged, human subscribers, your campaigns will yield better results, leading to stronger customer relationships and improved ROI.

Stay vigilant, adapt your strategies as new threats emerge, and prioritize the integrity of your email database to ensure long-term success.