Email Subject Line Tester
Test subject lines for length, spam words, emoji rendering and Gmail / Outlook preview.
About this tool
Score your subject line on length (40-50 chars sweet spot for mobile), spam-trigger words, emoji rendering risk, and ALL CAPS / excessive punctuation. Also previews how it'll render in Gmail and Outlook inbox lists.
Subject line rules that matter
- • 40-50 chars is the mobile sweet spot
- • 1 emoji can lift opens 5-15%; 3+ usually trips spam filters
- • Single exclamation is fine; 2+ exclamation marks signal spam
- • ALL CAPS words trigger demotion in many filters
- • Personalisation tokens lift opens 10-25% when used right
Frequently asked questions
What's the ideal subject line length? +
40-50 characters is the mobile sweet spot, fits cleanly in Gmail, Apple Mail and Outlook mobile inbox previews without truncation. Under 30 looks empty; over 70 truncates. Pair with a strong preheader to extend the visible message.
Do emojis hurt or help? +
One well-chosen emoji typically lifts opens 5-15%. Two is the upper limit before diminishing returns. Three+ trips spam filters and looks like clickbait. Position matters, at the start = pattern interrupt, at the end = visual close.
What spam words should I avoid? +
Heavy hitters: 'free money', 'act now', 'guaranteed', 'risk free', '100% free', 'cash bonus', '$$$', 'urgent'. Light hitters (acceptable in moderation): 'free', 'discount', 'save', 'offer', 'sale'. Use the spam-trigger checker in this tool to scan your specific subject.
Should I use personalisation tokens? +
Yes, {{first_name}} or city-based tokens lift opens 10-25% when the data is clean. But always provide a fallback ('Hi {{first_name|there}}') so 'Hi ,' never sends. Test with a few fallback-only subscribers before scheduling a campaign.
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