Free Email Subject Line Tester: Check Length, Spam Words and Preview
Instead of a mystery "AI score", this tester shows you exactly which checks your subject line passes and why each one matters. Paste a subject line and a preheader (the preview text after it) and see how it may look in a mobile inbox.
What these checks mean
- Length: many mobile apps show roughly 30 to 50 characters of a subject, so front-load the point.
- Spam-style words: mailbox providers look at many signals, such as sender reputation, authentication and engagement. Wording is only one of them.
- Preheader: a free second line that most senders waste.
Better than any score: test it
Most email platforms let you A/B test two subject lines on part of your list. Change one thing at a time, wait for a meaningful sample, and track opens together with clicks. Open rates can be inflated by privacy features in some mail apps, so clicks are the more reliable signal.
Be honest in the subject line
Subjects that mislead (for example a fake "Re:" or a false promise) break anti-spam laws in many countries, such as the US CAN-SPAM Act, and cost you trust. Write what the email actually contains.