The Most Important Factor: Mobile Preview Length
Over 60% of emails are opened on mobile. Most mobile email clients display 30–40 characters of the subject line before truncating. Write your most compelling words first. The remaining secondary message can go in the preheader text.
Power Words That Increase Open Rates
Words that create urgency: "Last chance", "Ends tonight", "48 hours left". Words that create exclusivity: "Exclusive", "Members only", "Invitation". Words that arouse curiosity: "You won't believe", "This surprised us", "The real reason". Words of value: "Free", "Save", "% off". Use sparingly — overuse looks spammy.
What Triggers Spam Filters
Avoid: Excessive punctuation (!!!), ALL CAPS subject lines, phrases like "Buy now", "Click here", "Free gift", "Act now", "No cost". Spam filters also score on sender reputation, HTML-to-text ratio, and unsubscribe compliance.
A/B Testing Subject Lines
Always test with ≥1,000 recipients per variant for statistical significance. Test one variable at a time: length vs length, emoji vs no emoji. Measure unique open rate, not total opens (more reliable). Run tests for at least 4 hours before picking a winner.