What’s the difference between content-based and behavioral filtering?

Your emails are landing in spam and you're not sure why. Is it something you wrote? Or is it the way you've been sending over time? That question points to two different filtering systems, and knowing which one is at play changes how you fix it.

Content-based filtering looks at what's inside the email. Subject lines, body copy, HTML structure, links, images, the ratio of text to images. Filters like Spamhaus and the scanning engines built into Gmail or Outlook check every message for patterns that match known spam. This type of filtering can catch you on a single send. One badly worded subject line, one link pointing to a flagged domain, one HTML tag that looks like a cloaking trick, and you're filtered.

The upside is that content-based issues are usually diagnosable fast. You can test them. Change the subject line, clean up the HTML, swap a link, send again. You'll know within a campaign or two whether that was the issue.

Behavioral filtering is different. It doesn't care as much about what you said. It cares about what you've done, consistently, over time. It looks at your sender reputation, engagement history (are people opening, clicking, deleting without reading, marking as spam?), and whether your sending patterns look normal or suspicious. This builds up over weeks and months. It also unravels over weeks and months.

Now the tricky part with behavioral filtering is that you can't fix it with a single tweak. If your reputation has dropped because you've been mailing a stale list, a reworded subject line won't save you. The inbox providers have already formed an opinion about you.

Here's how to tell which one is hitting you.

• If it's sudden and affects specific campaigns, think content. Look at what changed in that email. Subject line, links, image-to-text ratio, anything new.
• If it's gradual and affects everything you send, think behavioral. Your reputation has drifted. Check your bounce rates, complaint rates, and open rates over the last 60 to 90 days for a trend.
• If you're a new sender or recently changed ESPs, it's almost certainly behavioral. You haven't built enough positive engagement history yet.
• If you can send to one domain fine but not another, think about whether the stricter domain is using different content filters or has a reputation signal you don't have elsewhere.

Modern spam filters use both systems at once, and they interact. A strong reputation can carry you through content that would otherwise trigger a flag (think of a trusted friend sending you something that looks like spam but you open it anyway). A poor reputation makes even clean content suspicious. That's why diagnosing the root cause matters before you start changing things randomly.

If you want a place to start, check your domain with our free Blocklist Checker to rule out a hard reputation block. Then look at your sending data. If something doesn't add up, the SOS hotline is free and we'll help you work through it.