The 'MailBlockKnownSpammer' bounce message indicates that the receiving mail server considers the sending IP or domain to have a history of sending spam. This can stem from various factors including poor sending practices, low engagement, spam complaints, being listed on blocklists (RBLs), or sudden increases in email volume. Resolution involves checking and improving sender reputation, ensuring proper email authentication (SPF, DKIM, DMARC), maintaining a clean email list, segmenting lists for targeted sending, separating transactional and marketing emails, warming up new IPs, monitoring feedback loops, and avoiding spam trigger words in email content. Contacting the recipient's email provider or security filter provider for delisting may also be necessary.