Emails land in Outlook and Hotmail spam folders due to a complex interplay of factors, primarily related to sender reputation, email authentication, content quality and user engagement. Microsoft heavily weighs user interaction and feedback when filtering emails. Maintaining a positive sender reputation involves ensuring proper email authentication (SPF, DKIM, DMARC), monitoring IP and domain reputation through Microsoft's SNDS, and keeping complaint rates low via JMRP. Content should be clean, valuable, and avoid spam trigger words or excessive links. Permission-based marketing, consistent engagement, and regular list cleaning are crucial for improving inbox placement. Technical aspects like email headers, consistent sending IPs, and responsive email design also play a significant role. User settings in Outlook can affect whether emails are marked as spam.