Hotmail's email blocking is multifaceted, stemming from issues within the sender's network infrastructure, email content, reputation, authentication practices, and recipient engagement. Microsoft extensively examines sender networks, scrutinizing third-party providers, image hosting, and shared resources. Maintaining impeccable list hygiene by avoiding spam traps and ensuring valid email addresses is critical. Proper email authentication with SPF, DKIM, and DMARC is a necessity. Sender reputation is paramount; low engagement, high complaint rates, and manual emails without unsubscribe options can lead to severe deliverability problems. Monitoring IP reputation via SNDS and JMRP, consistent sending patterns, and strong opt-in practices are also essential.