Subject lines are the cheapest variable to test in cold email and the most over-engineered. After A/B testing thousands of variants, almost all winners share three traits: short, specific, and lowercase. Here are the patterns that consistently beat 50% open rates in B2B inboxes.
The rules behind every winner
- 3–6 words. Mobile clients cut at ~40 characters.
- Lowercase. Title case reads like marketing.
- No emoji. Hurts deliverability and B2B credibility.
- Specific to them. Generic = unopened.
- No "RE:" tricks. Gmail flags fake replies.
9 patterns that work
- quick question about [their thing]
- idea for [their team]
- [mutual context], thoughts?
- [specific observation about their company]
- {first name}, 2 minutes?
- re: [their recent post / news] (only if real)
- worth a look?
- noticed [thing] on [their site]
- thought of you (use sparingly)
Patterns that don't work anymore
- "Free" anything, flagged on every major provider
- "Increase your revenue by X%", reads like marketing
- ALL CAPS or excessive punctuation
- "Last chance" / "Final notice", manipulative, low trust
- Long descriptive sentences, looks like newsletter
How to test correctly
Test one variable at a time, with a sample size of at least 200 sends per variant. Anything smaller is noise. Run subject line tests for at least 5 business days before declaring a winner.
Subject lines are 30% of cold email performance. The other 70% is body, sequence and deliverability, covered in our complete cold email guide. Want us to run the testing for you? Book a fit call.