Subject Line Science

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.

What Gets Open Rates Wrong

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.

Characteristics of High-Performing Subject Lines

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.

Subject Line Templates That Work

• "[Company] + [Your Company]" — Simple, bilateral, not presumptuous
• "idea for [specific thing they'd care about]" — Teases value, creates curiosity
• "[Mutual customer or relevant company]" — Social proof by reference
• "[Number] [outcome relevant to them]" — Data-led, specific

What to Test

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.