A list of email addresses, domains, or IP addresses that have been flagged or blocked by email service providers, spam filters, or marketing platforms due to previous instances of spamming, malicious activity, or non-compliance with email best practices. When an email address or domain is blacklisted, emails sent from it may be automatically filtered into spam folders or rejected entirely by recipients' email servers. For businesses engaging in cold emailing, maintaining a clean reputation and avoiding blacklisting is crucial to ensure that messages reach their intended recipients and achieve their outreach goals. Regularly updating and monitoring blacklists helps prevent deliverability issues and protects the sender’s domain reputation.