What is a spam filter?
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Every time an email arrives at someone's inbox, something is quietly deciding whether you get in or get blocked. That something is a spam filter.
A spam filter is software that screens incoming email and decides whether a message belongs in the inbox, the spam folder, or should be rejected outright. It's running on the receiving side, meaning the recipient's email provider makes the call, not you.
Every major inbox provider runs one. Gmail, Outlook, Yahoo Mail, and corporate mail servers all use their own filtering systems, each with its own rules and thresholds. That's why the same email can land in the inbox at Gmail and get flagged at Outlook.
Filters look at a mix of signals to make that call. Authentication results (did SPF, DKIM, and DMARC pass?), sender reputation (has this IP or domain caused problems before?), content patterns (what's actually in the email?), and engagement history (do people open and click, or immediately delete?) all factor in. No single signal makes the decision. They're combined into an overall score, and if that score crosses a threshold, your email doesn't make it through.
The tricky part for senders is that you can't see the filter's reasoning directly. You only see the outcome. That's why understanding what filters look at matters so much, and why things like how spam filters make their decisions is worth digging into once you have the basics.
If your emails are landing in spam and you're not sure why, a good first step is checking whether your domain is on a blocklist. That's one of the fastest signals a spam filter uses to make up its mind.
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