The subject line decides whether a cold email gets opened or deleted — often before the recipient reads a single word of the body. Our free Subject Line Tester scores your subject line in seconds and flags the habits that quietly tank open rates.

What it checks: character length (subject lines that are too short or too long tend to underperform), spam-trigger words and phrases that email providers commonly flag (words like “”free,”” “”guarantee,”” “”act now””), excessive capitalization, and overuse of exclamation points.

How to use it: paste your draft subject line and click Test. You’ll get a score out of 100 along with specific flags explaining exactly what to fix — not just a number with no explanation.

Why this matters for outbound: a subject line stuffed with spam-trigger words doesn’t just get ignored by a human — it can also increase the odds of landing in a spam or promotions folder before anyone sees it at all, which quietly kills reply rates on campaigns that otherwise had great targeting.

Best practice guide: aim for 20–60 characters, write like a colleague rather than a marketer, avoid ALL CAPS and multiple exclamation points, and test two or three variations against each other rather than guessing which one works.

Who this is for: SDRs, growth marketers, and founders running their own outbound who want a quick gut-check before hitting send on a sequence that will reach hundreds of prospects.

Run your subject line through the tester above before your next send — it takes ten seconds and could be the difference between an open and a delete.