What this tool does
The Email Subject Line Tester takes the body of an email and returns five subject-line variations, each using a different hook. Ranked strongest-to-weakest. The first one is the generator's bet on which gets the most opens; the others are alternatives that lean on different psychological angles.
The five hooks
- Curiosity — opens a loop the reader needs to close. "Why we stopped using Slack." Works when there's a real story behind the email.
- Specificity — leads with a concrete number, name, or fact. "20 min on the Q3 launch?" Works in B2B where vague is the default and specific stands out.
- Urgency — names a deadline or constraint. "Tuesday or Wednesday?" Works when there's a real time-box, fails when it's manufactured.
- Value — names what the recipient gets. "Quick way to cut your build time in half." Works when you have a credible value claim.
- Contrarian — states the opposite of conventional wisdom. "Why long cold emails fail." Works when you've earned the contrarian position.
Why short subjects win
Most inboxes truncate at around 60 characters; mobile truncates earlier, often at 35-40. The generator caps every subject at 50 by design. A 75-character subject that says exactly the right thing still loses to a 40-character subject that says almost the same thing — because the 75-character version gets cut off mid-word and looks broken in the recipient's preview pane.
What it won't do
This tool generates subject lines for one specific email body. It's not an A/B-testing platform — for that, you need a proper outbound tool that can send the same email with different subjects to a list and measure opens. It also won't fix a bad email. If the body is vague or off-target, a great subject just gets you opened by someone who then bounces. Subject lines drive opens; bodies drive replies.