What is content-based spam filtering?

You spend time writing an email, it looks clean, your list is solid, and it still lands in spam. Sound familiar? That's often content-based spam filtering at work. These filters don't care who you are. They read what you wrote and decide whether it looks like spam based on the message itself.

Content filters scan the whole package: subject line, body text, HTML structure, links, and image-to-text ratio. They're looking for patterns that have historically shown up in spam. Think phrases like "Act now" or "You've been selected," links that point to sketchy domains, HTML that hides text with white-on-white tricks, or emails that are almost entirely one big image with almost no text.

Here's a quick picture of what tends to raise a red flag:

• Trigger phrases in subject lines or body like "FREE!!!", "Click here to claim", "No credit card needed", or anything written in ALL CAPS.
• Suspicious links that redirect through multiple domains or use URL shorteners without a clear destination.
• Poor image-to-text ratio, meaning an email that's mostly images with almost no readable text. (Image-only emails are a consistent spam signal.)
• Broken or messy HTML that looks like it was generated by a tool trying to hide something.
• Attachments that match known malware delivery formats, like certain executable file types.

One thing content filtering does that reputation-based filtering can't is catch bad emails from new or unknown senders. There's no track record to check yet, so the filter has to go on content alone. It also helps flag accounts that were once legitimate but have since been compromised.

Content filtering doesn't operate in isolation. A sender with a strong reputation can still get flagged if the email looks suspicious. And someone with a poor sending reputation sending a clean email may still get blocked. The two layers work together.

If you want to see how your subject line reads to a filter before you hit send, our free subject line tester can catch common red flags before they catch you.