Emails suddenly going to spam in Outlook can be attributed to a variety of factors. These include changes in Outlook's filtering algorithms, blacklisting, sudden spikes in email volume, poor list hygiene, and low user engagement. Specific content within emails, such as certain words or excessive links, can also trigger spam filters. Maintaining a good IP address reputation, participating in feedback loops, and ensuring proper authentication (SPF, DKIM, DMARC) are crucial for preventing deliverability issues. It's also essential to monitor SNDS, header scores, and technical difficulties at Microsoft, as well as investigate potential 'list bomb' attacks.