SpamAssassin significantly affects email deliverability through its rule-based scoring system, where emails exceeding a defined threshold are flagged as spam. Factors influencing SpamAssassin scores include poor HTML coding, spam trigger words, broken links, lack of authentication (SPF, DKIM, DMARC), and sender reputation. Blacklisting severely impacts deliverability, as SpamAssassin consults blacklists. Proper email authentication, maintaining a positive sender reputation, and ensuring list hygiene are crucial for evading SpamAssassin filters. While triggering custom SpamAssassin rules might not lead to immediate blocking, it's wise to periodically review email practices. Testing emails with tools like GlockApps before sending them to a large audience, using dedicated IPs, avoiding URL shorteners and excessive capitalization, and carefully choosing TLDs can further improve deliverability.