Email Subject Line Tester
Your subject line decides whether the email gets opened or ignored. Type one in to score it on length, spam triggers, formatting and personalization — and preview exactly how it lands in the inbox.
What makes a subject line work
The best subject lines are short enough to survive the mobile cut, specific about the value inside, and free of the tired sales language that trips spam filters. Personalization and a touch of curiosity help — but clarity always beats cleverness. Use the score as a guide, then trust an A/B test on your own list for the final word.
Don't waste the preview text
The subject line shares inbox space with two things you also control: the sender name and the preview (preheader) text — the grey snippet shown right after the subject. Most senders leave it blank, so the inbox grabs whatever comes first in the email, often a wasted "View this email in your browser." Write a short preheader that extends the subject instead of repeating it, and use a recognisable sender name. Together they act as a second headline that can lift opens as much as the subject itself.
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Frequently asked questions
What is the ideal email subject line length?
Aim for about 28–50 characters. Mobile inboxes typically show 30–40 characters before truncating, so the most important words should come first.
What words trigger spam filters in subject lines?
Overused sales words like 'free', 'guarantee', 'act now', 'risk-free', 'cash', '100%' and excessive punctuation or ALL CAPS can raise spam scores. No single word will doom you, but stacking them hurts deliverability.
Should I use emoji in subject lines?
A single, relevant emoji can lift open rates by adding visual contrast, but overusing emoji looks spammy and may render inconsistently across email clients. Test with and without for your audience.