What are the pros and cons of using Salesforce Marketing Cloud's Custom List Detective?
Michael Ko
Co-founder & CEO, Suped
Published 17 Jun 2025
Updated 16 Aug 2025
8 min read
Managing email deliverability within Salesforce Marketing Cloud (SFMC) often involves navigating various built-in tools designed to optimize email campaigns. One such tool is List Detective, a foundational feature that automatically filters out known bad email addresses. This proactive scrubbing helps maintain list hygiene and improves overall sender reputation.
However, email environments are dynamic, and organizations sometimes face specific challenges like malicious sign-ups or targeted spam attacks. This is where Custom List Detective (CLD) comes into play. It extends the standard List Detective functionality, allowing users to define their own rules for blocking specific email addresses or domains.
The decision to implement CLD, especially for a client facing malicious activity, requires a thorough understanding of its capabilities and limitations. While it offers a valuable layer of defense, it is crucial to weigh its pros against its cons to ensure it aligns with broader email security and deliverability strategies.
Understanding Salesforce Marketing Cloud's List Detective
Salesforce Marketing Cloud's standard List Detective is a proprietary database that automatically identifies and filters out problematic email addresses. This process occurs whenever lists are imported into the system, ensuring that known invalid, abusive, or spam trap email addresses are not included in your sends. This helps maintain the health of your contact lists.
The system keeps information on email addresses and domains that can cause deliverability problems. This includes addresses that are known bounces, spam traps, or have other indicators of poor quality. By preventing sends to these addresses, List Detective helps to reduce bounce rates and protect your sender reputation from being negatively impacted.
It serves as a foundational layer of defense, automatically preventing engagement with contacts that are likely to harm your sending metrics. This automated filtering is a significant benefit for marketers, allowing them to focus on engaging legitimate subscribers without constant manual scrubbing of their lists. More details on its function can be found in the Salesforce help documentation.
The role of Custom List Detective (CLD)
Custom List Detective (CLD) builds on the standard List Detective by allowing you to define specific rules for email addresses or domains. This feature is particularly useful when you encounter patterns of malicious activity not yet covered by Salesforce’s proprietary database, such as specific bot-generated domains or frequently abused usernames.
CLD gives marketers greater control over their suppression lists. It enables you to add custom entries to block new or evolving threats, or, in some cases, to allow specific addresses that might otherwise be caught by the standard List Detective. This flexibility is key for adapting to unique challenges that your sending environment might face.
The ability to restrict subscribers based on their email username or domain provides a granular layer of control. For instance, if you notice a surge of suspicious sign-ups from a particular, obscure domain, CLD allows you to quickly add that domain to your blocklist (or blacklist) to prevent future sends. This is especially relevant when trying to mitigate impacts on sender reputation.
Implementing Custom List Detective offers several notable advantages, particularly for organizations grappling with specific or emerging threats to their email lists. It can be a vital tool in maintaining a healthy sending environment.
One of the primary benefits is the enhanced control it provides over your email suppression. You're not solely reliant on Salesforce's global list, but can proactively block domains or usernames that show signs of abusive behavior specific to your acquisition channels. This helps prevent sending to known problematic addresses that might not yet be on the standard blocklists, reducing bounces and spam complaints.
This leads to improved deliverability metrics. By keeping invalid or malicious email addresses off your sending lists, your campaigns are less likely to encounter hard bounces, spam traps, or high complaint rates. These improvements positively impact your sender reputation, making it more likely that your legitimate emails reach the inbox. It aligns with best practices for automated list management.
Key advantages
Proactive threat mitigation: Blocks newly identified spam domains or usernames before they can cause significant damage.
Improved list quality: Helps keep your subscriber lists clean by preventing sends to known malicious addresses.
Reduced complaint rates: Less likely to hit spam traps or engage with highly engaged detractors.
Better resource allocation: Avoids wasting sending capacity and budget on invalid contacts.
Cons of Custom List Detective
Despite its advantages, Custom List Detective is not a silver bullet and comes with its own set of limitations and potential pitfalls. Understanding these drawbacks is essential for setting realistic expectations and implementing complementary solutions.
One significant limitation is that CLD doesn't fully address sophisticated bot sign-ups. Malicious actors often use tricks like adding dots to Gmail addresses (e.g., john.doe@gmail.com vs. j.o.h.n.d.o.e@gmail.com), which are treated as the same address by the inbox provider but appear as different entries to a simple string-based blocker like CLD. This can lead to your lists still being polluted.
Additionally, CLD blocks both commercial and transactional messages. If an email address is on your custom blocklist (or blacklist), Salesforce will deny sending to it regardless of whether the email is a marketing newsletter or a critical password reset notification. This can inadvertently prevent important transactional emails from reaching legitimate users, impacting customer experience. It’s important to understand the reasons emails go to spam beyond basic blocking.
Limitations and considerations
Incomplete bot protection: Sophisticated bots can bypass CLD by using variations of legitimate email addresses or dynamic, single-use domains, leading to continued list pollution.
Lack of distinction: Applies to both commercial and transactional sends, potentially blocking critical communications unnecessarily. This can lead to marketing cloud mistakes.
UI quirks: Past issues with the user interface meant allow entries sometimes acted as block entries, requiring Salesforce support intervention.
Not a substitute for front-end validation: It's a reactive measure, not a primary defense against fraudulent sign-ups at the point of entry.
Complementary strategies for email security
To effectively combat malicious email activity and maintain high deliverability, it is essential to employ a multi-layered approach. Custom List Detective can be one part of this strategy, but it should be complemented by other preventative and reactive measures.
The most critical defense against bot sign-ups and list pollution is implementing robust front-end validation, such as CAPTCHA, on your web forms. This prevents bad actors from entering invalid or malicious email addresses into your system in the first place, reducing the need for downstream cleanup by tools like CLD. This proactive stance is always superior to reactive blocking.
While CLD is helpful for specific domain or username blocking, for broader list hygiene, consider integrating with third-party list cleaning services. These services often use more advanced techniques to identify invalid, risky, or low-quality email addresses beyond simple domain or username checks. Regularly cleaning your lists this way can significantly improve your overall email performance and reduce the chances of hitting spam traps.
Finally, continuous monitoring of your email campaigns for anomalies, bounces, and complaint rates is vital. Tools that provide insights into your email deliverability allow you to react quickly to emerging threats or issues, complementing the automated and custom blocking rules in SFMC.
Views from the trenches
Best practices
Always prioritize robust web form validation with CAPTCHA to prevent invalid sign-ups at the source.
Regularly review your Custom List Detective entries to ensure they are still relevant and effective for your specific threats.
Utilize List Detective primarily for blocking specific malicious domains or patterns identified within your email logs, rather than as a broad list hygiene tool.
Common pitfalls
Assuming Custom List Detective will solve all bot-related or malicious activity issues without other protective layers.
Not understanding that CLD blocks both commercial and transactional sends, which could impact critical communications.
Relying on CLD for `allow` list functionality without verifying its behavior, as it has had historical bugs.
Expert tips
For advanced threat detection, consider using an email validation service that integrates with your signup forms.
If using Custom List Detective, clearly document why each entry was added and its intended purpose.
Consult Salesforce support for clarification on specific CLD behaviors, especially concerning 'allow' entries or send classification distinctions.
Expert view
Expert from Email Geeks says Custom List Detective blocks both commercial and transactional sends, as it prevents import for list-based sending and applies at send time for data-extension-based sending.
2022-04-01 - Email Geeks
Expert view
Expert from Email Geeks says Custom List Detective can block domains and certain usernames but does not effectively solve bot sign-ups that use Gmail dot variations.
2022-04-02 - Email Geeks
Balancing protection and performance with CLD
Salesforce Marketing Cloud's Custom List Detective offers a valuable layer of defense against specific unwanted email addresses and domains. It empowers marketers with additional control, allowing them to block emerging threats or fine-tune their suppression lists beyond the standard List Detective functionality.
However, it's crucial to recognize that CLD is a component of a larger email security strategy, not a standalone solution. Its limitations, such as not fully preventing sophisticated bot attacks or blocking transactional emails, highlight the need for a comprehensive approach. This approach should include robust front-end validation on signup forms and consistent list hygiene practices.
By understanding both the strengths and weaknesses of Custom List Detective, organizations can integrate it effectively into their existing email ecosystem, ensuring healthier lists, improved deliverability, and better protection against malicious activity.
Salesforce Marketing Cloud's standard List Detective is a proprietary database that automatically identifies and filters out problematic email addresses. This process occurs whenever lists are imported into the system, ensuring that known invalid, abusive, or spam trap email addresses are not included in your sends. This helps maintain the health of your contact lists.
