Why It Matters
Spam filters are what stand between your email and your recipient's inbox. Even if you're a totally legitimate sender, a misconfigured email, a bad subject line, or a reputation dip can trigger these filters. Understanding how they think helps you avoid their tripwires. About 20% of legitimate marketing emails never reach the inbox — filters are the primary reason.
How It Works
Filters evaluate emails on multiple dimensions simultaneously. They check sender authentication (SPF, DKIM, DMARC), look up the sending IP and domain against blocklists, analyze the content for spam patterns, and factor in how previous recipients have interacted with your messages. Each factor contributes to a "spam score," and if it crosses a threshold, your email gets filtered. Different providers weigh these factors differently — what passes at Yahoo might fail at Gmail.
Quick Tips
- Avoid image-only emails — filters can't read text inside images, which looks suspicious.
- Keep your text-to-link ratio healthy. More than 2-3 links per 100 words can trigger filters.
- Use a real reply-to address. "noreply@" emails see higher spam filtering rates.
- Test every campaign through a spam-checking tool like Mail Tester or GlockApps before you send.