What Makes a Great Cold Email Subject Line?
Your subject line has one job: get the email opened. Nothing else matters until that happens.
After analysing over 10 million cold emails sent through the Leads Genie platform, we found that the highest-performing subject lines share five characteristics:
1. Short — 3–7 words outperform longer subject lines by 21%
2. Specific — Mentioning the prospect's company or name increases open rates by 34%
3. Conversational — Lowercase performs better than Title Case
4. Curiosity-driven — Questions and incomplete thoughts drive clicks
5. No spam words — Free, guarantee, urgent, limited time all trigger filters
The Top Performing Subject Line Patterns
1. The Direct Question
*Average open rate: 64%*
- "Quick question about [Company]"
- "How are you handling [problem]?"
- "Still the right person for [topic]?"
2. The Personalised Observation
*Average open rate: 61%*
- "Congrats on the Series B"
- "Just saw your post about [topic]"
- "Your [product feature] caught my attention"
3. The Social Proof Hook
*Average open rate: 58%*
- "How [Competitor] books 50 demos/month"
- "[Mutual connection] thought I should reach out"
- "What [Similar Company] did to 3x their pipeline"
4. The Curiosity Gap
*Average open rate: 56%*
- "This might be useful for [Company]"
- "Thought you'd find this interesting"
- "One thing about your outreach"
5. The Blunt Ask
*Average open rate: 52%*
- "Quick intro?"
- "15 minutes?"
- "Worth connecting?"
Subject Lines to Avoid
These patterns consistently underperform or trigger spam filters:
- "Following up on my last email" — marks you as a spammer
- "RE: [fake thread]" — deceptive, destroys trust
- "FREE: [anything]" — spam trigger
- "URGENT: [anything]" — spam trigger
- Long subject lines (10+ words) — get cut off on mobile
A/B Testing Your Subject Lines
Use Leads Genie's built-in A/B testing to test subject lines on real campaigns. Our recommendation:
- Test one variable at a time (question vs. statement)
- Run each variant on at least 200 contacts before declaring a winner
- Look at reply rate, not just open rate — a subject line that gets opens but no replies is a vanity metric
The 3-Second Rule
If you can't explain what makes your subject line compelling in 3 seconds, rewrite it. Your prospect makes the open decision in less than a second. Be clear, be human, be specific.