Online Tool Store
Email Subject Line Tester
Preview your email subject and preview text on mobile and desktop, and check for length issues and spam trigger words. Runs entirely in your browser.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
Mobile inbox preview
Desktop inbox preview
Subject length
Spam trigger words
Overall score
How the email subject line tester works
- Enter your subject line and preview text (preheader).
- See mobile and desktop inbox previews showing exactly what recipients will see.
- Review length, spam trigger words, and an overall score with actionable tips.
FAQ
How long should an email subject line be?
Aim for under 50 characters - most mobile inboxes truncate subjects around 30-40 characters, so front-load the important part.
What are spam trigger words?
Phrases like "free", "act now", or "100% guarantee" are commonly flagged by spam filters, especially in combination with other spammy signals like ALL CAPS or excessive punctuation.
What is preview text (preheader)?
The short snippet of text shown next to or below the subject line in most inbox views. It's a second chance to earn a click, so it shouldn't just repeat the subject.
Does a high score guarantee good deliverability?
No - this checks common best-practice signals in the subject and preview text itself. Actual deliverability also depends on sender reputation, authentication (SPF/DKIM/DMARC), and list health.
How we compare
| Feature | Online Tool Store | Eyeballing it | Full email marketing platform |
|---|---|---|---|
| Mobile + desktop preview | ✓ | ✗ | ✓ |
| Spam trigger word detection | ✓ | ✗ | ✓ |
| No email list required | ✓ | ✓ | ✗ |
A quick pre-send check before drafting your subject in a full email platform.