Email Subject Line Tester
Paste a subject line and preheader to check length, spam trigger words, emoji use and how the line truncates in real inboxes.
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This tool runs entirely in your browser. Nothing you type is uploaded, logged or stored.
Optimising Open Rates, Spam Filters and Inbox Display
Subject lines are judged in a fraction of a second next to dozens of competing messages, and display space is the first constraint. Mobile clients typically show 30–40 characters, desktop clients 60–70, and smartwatch notifications fewer than 25. Front-load the meaningful words: a subject that reads perfectly at full length but truncates to 'Your monthly update on…' has wasted the only part most subscribers see.
The preheader is the second half of the pair and the most under-used asset in email. Most clients display it directly after the subject, giving you another 40–90 characters to extend the hook rather than repeating it. Leaving it empty lets the client pull whatever text appears first in the HTML — often 'View this email in your browser', which reads like a template someone forgot to finish.
Spam filtering today is driven far more by sender reputation, authentication and engagement than by keyword lists, but the classic triggers still correlate with poor performance: all-caps words, multiple exclamation marks, currency symbols repeated for emphasis, and urgency clichés. Combine them and filters raise the score. Get the fundamentals right first — SPF, DKIM and DMARC configured, a consistent sending domain, clean list hygiene and prompt unsubscribe handling — then keep the subject specific, honest and human. Test one variable at a time with a genuine A/B split, and judge on clicks and replies rather than opens, which privacy-protection features now inflate unreliably.
Frequently asked questions
Roughly 30–50 characters, or six to eight words. That fits most mobile inboxes in full while still leaving room for a specific, concrete hook.