Email Subject Line Tester
Check subject length and spammy wording.
Use the Email Subject Line Tester tool
What this email subject line tester does
Email Subject Line Tester runs a set of readable rules over your subject line and gives a 0–100 heuristic score. It flags excessive length, all-caps words, multiple exclamation marks, common spam trigger phrases and missing personalisation, and shows the character count next to mobile and desktop preview widths.
The score is not a spam-filter result and cannot predict open rates. Mail providers weigh sender reputation, engagement and content, and audiences differ. Use the suggestions to remove obvious problems, then test real subject lines with a real audience.
Why use it
- Catch a subject line that will be truncated on a phone.
- Remove wording that looks like a spam pattern.
- Compare two versions objectively before choosing one.
How to use it
- 1Paste one subject line.
- 2Read the heuristic score and the individual warnings.
- 3Shorten or rewrite where the hints suggest it.
- 4Test the revised line with your own audience.
Worked examples
Shouty line
Input
FREE!!! Best offer ever!!!
Output
Warnings for all caps, repeated exclamation marks and spam trigger words, with a low score.
Specific line
Input
Your October invoice is ready
Output
A short, specific line that fits preview widths and scores well on the rules.
Common mistakes
- Treating the score as an open-rate forecast.
- Adding a first name to an otherwise vague subject.
- Optimising for length while losing the specific reason to open.
Frequently asked questions
What is a good length?
Around 30–50 characters usually survives mobile previews. The right length depends on the client and audience.
Does it check deliverability?
No. It checks wording patterns only. Deliverability depends on authentication, reputation and list quality.
Why is my score low despite good wording?
The rules are deliberately simple. Read the individual warnings rather than relying on the number.
Is my subject uploaded?
No. The analysis runs locally.