IPs and domains are suddenly blacklisted by Spamhaus due to various factors, including direct spam activity, compromised systems, poor email list hygiene, sudden changes in sending patterns, shared hosting issues, and negative IP reputation. Spamhaus's DBL, SBL, and XBL identify entities involved in spam, malware, and related activities. To prevent blacklisting, it's crucial to secure systems, maintain clean email lists, implement email authentication (SPF, DKIM, DMARC), monitor sender reputation, and understand Spamhaus's listing criteria and sophisticated filtering methods.