What are heuristic filters?

You've crafted a perfectly legitimate promotional email. Strong subject line, clear call to action, good offer. And it lands in spam. No bounce, no block, just silence. A heuristic filter probably caught it.

Heuristic filters work by running your email through a checklist of rules, each one based on patterns that spammers historically use. Every rule your message triggers adds points to a spam score. Cross a threshold and you're filtered out. It's less like a bouncer and more like a scoring system where enough small strikes add up to a rejection.

SpamAssassin is the most well-known example. It has hundreds of individual rules built up over two decades, and many mail servers still run it under the hood today.

Here's where legitimate senders get tripped up more often than you'd think:

• ALL CAPS in the subject line. Writing "FLASH SALE" or "FREE SHIPPING TODAY" reads as shouting to a heuristic rule. Even one aggressive word in caps can nudge your score upward.
• Too many images, too little text. If your email is mostly a banner graphic with a button, heuristic rules see an image-heavy message with almost no readable content. That pattern matches a lot of spam templates.
• URL shorteners. Using a shortener like bit.ly inside your email body hides the destination URL. Heuristic filters treat that as suspicious, because spammers use it to mask bad links.
• Hidden text. White text on a white background, tiny font sizes, or text with display:none in your HTML. Filters check for this. Even when it's left in accidentally by a template editor, it scores against you.
• Spammy phrases in the body. "Act now", "Limited time offer", "You've been selected" and similar urgency phrases each carry small scores. One or two is usually fine. Eight of them in a single email is a problem.
• Header irregularities. Mismatched From and Reply-To addresses, missing Message-ID, or unusual encoding can all trigger rules around header forgery patterns.

The useful thing about heuristic filters is that they're transparent. When a message gets flagged, you can actually see which rules fired and how many points each one added. That's a real advantage over machine learning filters, which make decisions in ways that are much harder to decode. With heuristics, the answer is right there in the score breakdown.

If you suspect a heuristic filter is the reason your email isn't landing where it should, start with your subject line and HTML. Those are the most common culprits. Our free subject line tester can flag high-risk phrases before you hit send. And if you want to dig into the raw email source to see exactly what a filter might be reacting to, the source analyzer is a good place to start.