Spam filters look for various indicators to classify an email as spam: 1. Suspicious Links: Links that redirect to malicious websites. 2. Unusual Attachments: Attachments that could contain malware. 3. High Image-to-Text Ratio: Emails with more images than text are often flagged. 4. Spammy Keywords: Words like "free", "guaranteed", and "urgent". 5. Incorrect HTML: Poorly coded HTML can be a red flag.