Email Subject Line Tester

How to use it

1. Enter your subject line and the preheader (the preview text shown after the subject in most inboxes).
2. Optionally set a sender name and a sample first name to fill personalisation tokens in the preview.
3. Read the checklist and the approximate mobile and desktop previews.
4. Add up to four more variants and compare them side by side before choosing which to test.

What each check looks for

• Length is counted in characters as you see them, so an emoji or accented letter counts as one.
• Mobile inboxes usually show about 35–40 characters of a subject and desktop inboxes about 60. These are approximations: the real cut-off depends on the app, screen width, font and the letters themselves.
• Emoji are counted individually, including combined emoji such as flags and family groups. They can display differently between apps, and some older clients show them as boxes.
• Capitals: the share of letters in upper case, plus any all-caps words of four or more letters. Exclamation and question marks are counted, and repeated punctuation is flagged.
• Spam-trigger phrases come from a short list shown in full on the page. Matching phrases are shown so you can judge them in context.
• Personalisation tokens such as {{first_name}}, *|FNAME|* and %%FirstName%% are detected, and tokens that look broken (an unclosed {{ or *|) are flagged. A sample value can fill first-name tokens in the preview.