Spam filters don't work the way most cold emailers think. Understanding the actual mechanisms helps you write emails that reach inboxes.
How Modern Spam Filters Work
| Layer | What It Checks | Impact |
|---|---|---|
| Connection | IP reputation, patterns | High |
| Authentication | SPF, DKIM, DMARC | High |
| Content Scoring | Text, links, formatting | Medium |
| Behavioral | User engagement history | High |
| Machine Learning | Pattern recognition | Very High |
Each layer assigns a score. The cumulative score determines inbox placement.
Content-Based Triggers
High-Risk Phrases
- "Act now" / "Limited time"
- "Free" with exclamation marks
- "Guaranteed" / "No risk"
- "Click here"
- Currency symbols in subjects ($$$)
- ALL CAPS
Context matters: "Free consultation" scores differently than "FREE!!! MONEY!!!"
Formatting Red Flags
- Excessive punctuation (!!!, ???)
- Random capitalization
- Image-heavy emails
- Hidden text
- Broken HTML
Link Problems
- URL shorteners (bit.ly)
- Mismatched anchor text and destination
- Too many links
- Links to newly registered domains
Safe practices: Use your own domain, match anchor text to destination, limit to 1-2 links.
Header-Based Triggers
Authentication Failures
- SPF hard fail from unauthorized IP
- Invalid DKIM signature
- DMARC policy violations
Suspicious Patterns
- Mismatched From and Reply-To domains
- Missing Message-ID
- Incorrect timestamps
Behavioral Signals
Positive: Opens, replies, moving from spam to inbox, adding to contacts
Negative: Marking as spam, deleting without reading, blocking sender
Suspicious Patterns: Sudden volume increases, burst patterns, identical content to many recipients
Common Cold Email Mistakes
The "Salesy" Email
<pre><code>Subject: AMAZING Opportunity!!! Hi there, INCREDIBLE offer that will 10X your business GUARANTEED! Click here NOW for FREE consultation worth $500!!!</code></pre>Issues: Excessive punctuation, ALL CAPS, pressure language, "click here" text.
What Works Instead
<pre><code>Subject: Question about [specific thing] Hi [Name], [One sentence showing research] [One sentence about why you're reaching out] [One clear question] [Your name] [Simple signature]</code></pre>Technical Best Practices
- Valid SPF, DKIM, DMARC
- Plain text or simple HTML
- One link maximum
- No attachments on first email
- Professional signature without images
Key Takeaways
- Authentication is foundational - SPF, DKIM, DMARC must be correct
- Behavior outweighs content - Engagement history matters more than word choice
- Reputation compounds - Every email affects future deliverability
- Test before sending - Check content and deliverability first
Test your emails: Use our Email Spam Checker to analyze content for triggers, then test delivery with our Deliverability Test.