A red filter result in Microsoft's SNDS indicates that SmartScreen identifies a high percentage of emails from a given IP as spam. While the SNDS color codes reflect SmartScreen's view, they aren't always perfectly accurate and don't guarantee deliverability issues. A red filter suggests potential problems with deliverability, particularly for Outlook.com and Hotmail, and warrants investigation of content, reputation, and sending practices. SmartScreen evaluates various signals, including content, links, and sender reputation. A good IP/domain reputation can mitigate content issues. Proper authentication (SPF, DKIM, DMARC), list hygiene, engagement rates, and bounce rates are all essential factors. It's recommended to correlate SNDS data with complaint rates and inbox placement testing.