Salesforce Marketing Cloud's Custom List Detective (CLD) helps prevent sending to invalid, role-based, or spam-likely addresses, blocking both commercial and transactional emails. It can block specific domains and usernames, aiding deliverability. However, it's not fully effective against sophisticated bot signups, and third-party email validation services are often recommended as a more robust alternative. CLD incurs costs based on lookups, and requires proper configuration, regular rule updates, and monitoring to maximize effectiveness. Integrating CLD with other email marketing tools is recommended. There was a past UI bug, and maintaining overall email data quality remains crucial.