About the Email Subject Line Tester
The subject line and preheader are all most people see before deciding to open. This email subject line tester shows both in desktop and mobile inbox mockups so you can see where text is likely to be cut off. It checks length, flags ALL CAPS words and excessive punctuation such as !!! or ???, highlights common spam-trigger phrases and detects personalisation tokens such as {first_name} that may display raw if the merge fails.
Spam filters today rely far more on sender reputation and authentication than single words, so treat phrase warnings as a prompt to reread, not a rule. A clear, specific subject usually beats a clever one. Everything is checked in your browser.
How to use it
- Type your subject line and preheader text.
- Compare desktop and mobile inbox previews.
- Review warnings on length, caps, punctuation and phrases.
- Rewrite and A/B test promising versions in your email platform.
Frequently asked questions
How long should an email subject line be?
There is no fixed rule, but mobile inboxes show fewer characters than desktop, so put the key words first. The preview shows where your text is likely to be cut on each layout. Test shorter and longer versions with your own audience.
What is preheader text?
Preheader text is the short snippet shown after or below the subject line in many inboxes. If you do not set it, email clients often pull the first text in your email, such as View in browser. Use it to add a reason to open.
Do spam trigger words send email to spam?
Single words rarely decide placement on their own. Filters weigh authentication, sender reputation, engagement and complaint rates heavily. Still, heavy use of salesy phrases, caps and punctuation can look spammy to readers, which affects engagement. Write for the reader first.