What makes a subject open
The subject is the gate to your email. You have 30-50 characters and a fraction of a second for the recipient to decide open or archive. The highest-performing subjects aren't the cleverest — they're the clearest. If the reader has to think, they don't open.
The 4 levers that drive opens
- Curiosity: leave an open question or unexpected statement. "A question about your CAC".
- Concrete benefit: the outcome the recipient wants. "30% less CAC in 60 days".
- Urgency or time: a date or window. "Before the 31st — only if it fits".
- Personalization: name of the recipient or their company. "Acme + a 2-minute idea".
What no longer works
- Fake "Re:" or "Fwd:". Lower replies once recipients catch on — and they do.
- "Last chance for X". If you said it three emails in a row, it wasn't the last.
- "Important: read this now". Sounds like phishing.
- ALL CAPS. Spam rate goes up.
- "How X changed my life". Buzzfeed-style died a decade ago in B2B.
How to A/B test subjects
If you send more than 100 emails per campaign, A/B testing is mandatory.
- Generate 5-10 subjects with different angles.
- Split your list evenly and send each subject to one group.
- Wait at least 48 hours for opens to stabilize.
- Pick the winner by open + reply, not opens alone. High open + low reply means the subject is overpromising.
- Iterate the winner with light variations to find the ceiling.
Operational best practices
- Most important in the first 35 characters (mobile).
- Avoid exclamation marks (max one).
- Don't use spam triggers: "free", "guaranteed", "100%", "$$$".
- Lowercase-only often works in modern cold outbound.
- Subject ↔ body coherence. If the subject promises X, the first paragraph delivers X.