Free tool
Free Email Subject Line Tester
Paste a subject line and get an instant score with the spam words, length problems, and formatting issues that hurt your open rate - plus a live preview of how it shows on phone and desktop. It runs entirely in your browser.
Runs entirely in your browser. Nothing is uploaded or stored.
How it works
Your subject line is the single biggest lever on whether an email gets opened. This tester grades the line you already have: it counts characters and words, flags spam-trigger words that route you to the promotions tab or spam folder, catches ALL-CAPS shouting and exclamation-mark abuse, and rewards the levers that lift opens - a specific number, a question, honest specificity.
How to use it: paste or type your subject line. You get a 0-100 score and a letter grade, a stat row (characters, words, spam words, ALL-CAPS words), a plain-English list of exactly what to change, and two inbox previews - phone and desktop - so you can see where the line gets truncated before you send. Most mobile clients cut off around 33 characters, so the front of your line has to earn the open.
Aim for 28 to 50 characters, zero spam words, at most one exclamation mark, and no accidental merge tags like {first_name} left showing. Fix the warnings first, then re-test. Pair it with our subject-line generator when you want fresh options, and the spam checker for the full email body.
This is the same discipline AutoEmail applies automatically: every AI-drafted reply and outreach subject is scored for spam and deliverability before it is staged for your approval - so nothing risky ever reaches a customer's inbox on your behalf.