What signals do spam filters look at?

Spam filters aren't just checking one thing when your email arrives. They're running through a whole checklist in milliseconds, pulling signals from four different categories before deciding where your message lands.

Authentication is the first thing they check. Did your email pass SPF, DKIM, and DMARC? These records tell the filter whether the sending server was actually authorized to send on your behalf. A failed authentication check is a serious red flag, and many filters will route you straight to spam before even looking at your content.

Reputation comes next. Filters look at the IP address your email came from, your sending domain, and any URLs inside the message. They cross-reference these against blocklists like Spamhaus and maintain their own internal reputation scores built up from past behavior. A clean IP with a good sending history gets a lot more benefit of the doubt.

Content covers everything the reader actually sees (and some things they don't). Subject line patterns, HTML structure, text-to-image ratio, suspicious phrases, and the quality of links inside the email all count. This is also where things like broken HTML or weird formatting can quietly hurt you.

Behavior is the signal that catches a lot of senders off guard. Filters track how real recipients respond to your email over time. High open rates and replies signal trust. High delete-without-open rates and spam complaints signal the opposite. Your past behavior feeds directly into future filtering decisions.

None of these four categories works alone. A great reputation won't save you if your authentication is broken. Clean content won't help much if your domain is on a blocklist. Filters combine all four signals together to reach a verdict on every single email you send.

Want to see where your setup actually stands? Our free SPF checker and blocklist checker are a good place to start.