Your subject line determines whether your email gets opened or deleted. In cold email, you don't have a sender reputation to rely on — the subject line does 90% of the work. This lesson covers what the data shows about what works.
Open rate is a misleading vanity metric in isolation. A subject line that generates 60% opens but 0% replies is worse than one that generates 35% opens and 12% replies. Optimize for reply rate, not open rate — and measure subject line performance by tracking reply rate by variant, not just opens.
Short: 4–7 words. Long subject lines get cut off on mobile and look like marketing emails.
Specific: Reference something about the recipient's company, industry, or situation. "Quick question" is generic; "HubSpot users at [Company]" is specific.
Lowercase: Title Case Feels Like Marketing Copy. lowercase feels like an email from a colleague.
No punctuation tricks: Excessive punctuation, brackets, or emojis signal bulk email and trigger spam filters.
Run A/B tests on subject lines with minimum 100 sends per variant. Test one variable at a time: length, specificity level, curiosity vs. direct. Document results in a shared swipe file so the team builds collective intelligence.