Score your subject lines for spam risk, preview them in Gmail, Apple Mail, and Outlook, and see exactly how they truncate on mobile.
Desktop inbox — ~60 chars visible for subject
The Email Subject Line Tester scores any subject line out of 100 before you hit send, then shows you exactly how it will look in Gmail, Apple Mail, and Outlook — so you catch problems while there is still time to fix them.
The first thing a subscriber sees is your sender name, subject line, and preview text sitting in a crowded inbox. Those few words decide whether your email is opened or ignored, yet most people write them blind and only discover issues after the campaign has gone out. This tool closes that gap. As you type, it breaks your score into four parts — length, spam risk, power words, and engagement — and explains every point in plain language. It flags classic spam trigger words like free, urgent, and winner, warns about ALL CAPS and stacked exclamation marks, counts characters against the limits that matter, and rewards proven open-rate boosters such as numbers, questions, emoji, and personalisation tokens. The live inbox previews show real truncation: roughly 60 characters on desktop, about 30 on mobile, so you can confirm your key words land before the cut-off on a phone screen. People use it to:
Everything runs locally in your browser — your subject lines, preview text, and sender name are never uploaded to any server, so unreleased campaign copy stays private. Remember that the score is a guide to best practice, not a guarantee of deliverability; your actual open rate still depends on your list quality, sender reputation, and the relevance of the message to each reader.
Scores of 70–100 indicate a well-optimised subject line — correct length, no spam triggers, strong power words, and engagement signals like numbers or questions. Scores of 50–69 are passable but can be improved. Under 50 means the subject has significant issues that will reduce open rates.
The ideal length is 41–50 characters — long enough to convey value, short enough to display in full on most desktop clients. On mobile, only ~30 characters are visible, so place your most important words first. Subjects under 20 characters often feel abrupt, while subjects over 60 characters are truncated on most clients.
Classic spam trigger words include: free, urgent, guarantee, winner, cash prize, click here, act now, buy now, limited time, and special offer. Using $ symbols, excessive exclamation marks, or ALL CAPS also increases spam scores. Modern filters use context, so one trigger word rarely causes filtering — but several together significantly raise your spam probability.
Aim for 41 to 50 characters. That range is long enough to convey a clear benefit yet short enough to display in full on most desktop clients, which cut off around 60 characters. On mobile only about 30 characters are visible, so put your most important words first. This tester scores length out of 25 points and shows live previews so you can see exactly where each client truncates your subject.
As you type, the tool scans for known spam triggers like free, urgent, guarantee, winner, and click here, and flags each one in the signals panel. It also warns when more than half your letters are uppercase, when you use more than two exclamation marks, or when you repeat the $ symbol. Reword flagged terms, drop the shouting and stacked punctuation, and your spam-risk score will rise toward the full 25 points.
A single well-chosen emoji can add visual contrast and lift open rates, which is why the tester awards engagement points when it detects one. Keep it to one or two at most, since a cluster of emoji can look like spam and may render inconsistently across clients. Always check the live inbox previews to confirm the emoji displays the way you expect before you send.
Preview text, also called the preheader, is the short snippet shown next to or below your subject line in the inbox. If you leave it empty, clients fall back to pulling the first words of your email body, which often looks messy. Enter your preheader in the second field and the tool shows it in the Gmail, Mobile, and Outlook previews so you can craft a subject and preview that work together to earn the open.