Free Email Subject Line Tester
Score your email subject line before you send. Get feedback on length, spam risk, and engagement potential.
PressEnter ↵to score
Your score will appear here
We grade length, spam words, capitalization, punctuation, personalization, numbers, questions, and power words. Everything runs in your browser.
How it works
3 quick steps from input to answer. No account, no install.
Back to the tool- 01
Type your subject line
Enter the subject line you plan to use in your campaign.
- 02
We score it across 8 factors
Length, spam words, capitalization, punctuation, personalization, numbers, questions, and power words.
- 03
Get a score with improvement tips
See what is working, what is not, and specific suggestions to improve open rates.
Why this matters
Your subject line determines whether your email gets opened or ignored. It is the first thing a prospect sees, and often the only thing they read before deciding. In cold outreach, a bad subject line means your carefully written email never gets read.
SendboxTest subject lines in real campaigns
Sendbox A/B tests your subject lines across live campaigns and shows you which ones drive the most opens.
A/B testing
Test multiple subject lines against each other in real campaigns
Open rate tracking
See which subject lines perform best across your campaigns
AI suggestions
Get AI-generated subject line alternatives based on your email content
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Good subject lines are 6-10 words, avoid spam triggers, create curiosity or urgency, and feel personal. They should be specific enough to set expectations but intriguing enough to earn the open.
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