How to identify and filter bot email addresses for email list hygiene?

Key findings

• Bot characteristics: Bot-generated email addresses often include random strings of characters, use disposable email domains, or legitimate domains like qq.com that are out of context for your target audience.
• Disposable domains: Using a list of known disposable email domains can help identify and filter out many unwanted sign-ups. Many marketers consider avoiding these to be a gold standard for hygiene.
• IP validation: Checking the submission IP addresses against lists of TOR exit nodes or proxies can help flag suspicious activity.
• Plus addressing: Email addresses with a '+' alias (e.g., email+tag@domain.com) can be used by legitimate users for tracking, but also by bots to bypass signup limits or exploit free trials.
• Proactive protection: Implementing protection directly on signup forms, such as CAPTCHA systems, can significantly reduce bot sign-ups. More information on protecting forms from bots can be found in our guide on how to protect email list signup forms from bots.

Key considerations

• Business context: While some domains like qq.com are legitimate, their presence on your list might indicate bot activity if your target audience is not in China. Assess what domains are atypical for your user base.
• Trade-offs of strict filtering: Be aware that aggressive filtering, such as blanket blocking of disposable domains or plus addresses, might inadvertently exclude a small percentage of legitimate users. Weigh the benefits of a cleaner list against potential lost legitimate sign-ups.
• Regular maintenance: Email list hygiene isn't a one-time task. Regularly review and scrub your list to remove suspicious or inactive contacts. You can learn more about this in our article on removing bad email addresses.
• Engagement signals: Accounts with no engagement history or suspicious patterns often indicate bot activity. Identifying bot subscribers frequently involves looking for these red flags.

Key opinions

• Disposable email lists: Many marketers rely on shared lists of disposable email domains (DEDs) to filter out bot sign-ups, viewing it as a highly effective measure for maintaining a healthy list.
• Form-level protection: There's a strong consensus that protecting signup forms directly, beyond just post-signup cleaning, significantly helps prevent bot entries.
• Contextual domain analysis: Even legitimate domains (like qq.com) can indicate bot activity if they don't align with the expected audience demographics or location.
• Balancing filtering: Marketers often debate the trade-off between strict filtering (which might remove a few legitimate, but perhaps less engaged, subscribers) and allowing potentially problematic addresses to remain on the list.
• Plus addresses for abuse: The use of '+' aliases is a known tactic by some users to repeatedly sign up for trials or bypass limits, leading some companies to block them entirely.

Key considerations

• Integration with ESP: Regularly syncing disposable email domain lists with your ESP (Email Service Provider) helps automate the disqualification of these addresses.
• Impact on customer experience: While preventing bots, consider how filtering mechanisms (e.g., CAPTCHAs, or blocking plus aliases) might impact the user experience for legitimate subscribers.
• Continuous monitoring: Email list hygiene is an ongoing process. Bots evolve, so your filtering methods must adapt. Learn more about ongoing prevention in our guide on how to prevent bot sign-ups.
• Understanding intent: Before blanket blocking, understand why certain patterns (like disposable domains or plus addresses) appear. Some users genuinely prefer them for privacy, while others use them for malicious purposes, as noted by Resend's audience hygiene tips.

Key opinions

• Behavioral vs. domain-based filtering: Experts suggest that focusing solely on domain blacklists is insufficient; analyzing submission IPs for signs of TOR exit nodes or proxies provides a more robust indicator of bot activity.
• Disposable address stance: A common expert recommendation is to not send emails to disposable addresses, as their use often indicates a lack of genuine interest in receiving marketing communications.
• Proactive form protection: Deliverability experts consistently advise implementing measures directly on signup forms to prevent bot entries from the outset, rather than solely relying on post-signup cleaning.
• Understanding '+' addresses: While useful for users, '+' aliases can be exploited. Experts highlight the necessity of balancing legitimate user needs with abuse prevention, particularly for free trials or gated content.
• Risk assessment: The potential negative impact of sending to bot-generated or low-intent addresses (e.g., spam traps, reduced engagement, blocklisting) generally outweighs the benefit of keeping such contacts.

Key considerations

• Data-driven decisions: Leverage available data, such as submission IP and domain context, to make informed decisions about which addresses to filter. This is critical for preventing fake or generated email addresses.
• Integrating validation tools: Implement real-time email validation APIs or tools at the point of signup to verify addresses immediately. For guidance on selecting the right tools, refer to best email address validation workflows.
• Policy enforcement: Establish clear internal policies regarding the acceptance of disposable domains and plus addresses, aligning them with business objectives (e.g., preventing free trial abuse).
• Long-term deliverability: Prioritizing a clean list directly contributes to higher deliverability rates and a positive sender reputation. As noted by Word to the Wise, maintaining email hygiene is foundational to avoiding blocklists and spam folder placement.

Key findings

• RFC compliance: Email addresses are governed by RFC standards (e.g., RFC 5322) that define their valid format. Deviations from these standards can indicate a malformed or bot-generated address.
• Role of DNSBLs: Many blacklists (or blocklists), particularly DNS-based Blocklists (DNSBLs), compile lists of IPs or domains associated with spam and bot activity. Understanding how these work is essential for list hygiene.
• Automated verification: Automated email verification processes involve syntax checks, domain validation (e.g., MX records), and often checking against lists of known disposable or risky domains.
• Spam trap detection: Documentation often details how spam traps are deployed by mailbox providers to catch senders with poor list management, including those who acquire addresses via bots. These traps are a critical reason to focus on list hygiene.

Key considerations

• Real-time validation: Technical documentation emphasizes implementing real-time email validation at signup to prevent invalid or bot-generated addresses from entering your database immediately.
• Leveraging APIs: Many email verification services provide APIs that can be integrated into forms or backend systems to perform comprehensive checks, including syntax, domain validity, and disposable domain detection. This is discussed in depth within resources about email validation APIs.
• Double opt-in: While not directly an identification method, documentation consistently recommends double opt-in as a crucial step to confirm subscriber intent and filter out many bot-generated or fake addresses that won't complete the confirmation process.
• IP reputation databases: Technical guides often reference public and private IP reputation databases that can be used to identify suspicious signup attempts originating from known malicious IPs. This information is vital for understanding your email domain reputation and preventing blocklisting.