Free toolPostWyse

Email Subject Line Tester

Paste a subject line and get a 0–100 score across length, spam triggers, emoji, capitalization, and curiosity signals — plus a live mobile inbox preview showing exactly where it truncates. No signup.

Subject line

Preheader (optional)

Mobile inbox preview

Your Brand

9:41 AM

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Subject score

100 / 100

Excellent — ship it

Length

36 chars — fits mobile and desktop

Spam triggers

None detected

Capitalization

Natural casing

Emoji

None — clean (one can lift opens, test it)

Curiosity signals

Has a number

Word count

8 words — scannable

The subject line is the whole campaign

No one reads an email they don't open. The subject line and preheader are the only things competing in a crowded inbox, and they decide your open rate before a single word of the email matters. Spending an extra two minutes here is the highest-leverage edit in email marketing.

This grader checks the mechanical things that quietly cost you opens — truncation on mobile, spam-trigger stacking, shouting in caps — and the human things that earn them: a number, a question, a specific promise, the word “you.” Treat the score as a pre-flight check, then A/B test the top two candidates.

Pair the subject with its preheader

The preheader is the preview text after the subject. Don't waste it repeating the subject or letting it pull “View in browser.” Use it to extend the hook — subject sets the tension, preheader adds the specific. Together they're one line of copy.

Want this on autopilot

PostWyse drafts subject lines in your voice, scores them, and ships the winner — then learns from your actual open rates which angles land with your list.

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Frequently asked

What makes a good email subject line?

The best subject lines are short (about 28–50 characters so they survive mobile truncation), specific, and curiosity-driven — often using a number, a question, or the word 'you.' They avoid spam triggers and all-caps, and they're backed by a preheader that extends the hook rather than repeating it.

How long should a subject line be?

Aim for roughly 28–50 characters. Mobile inboxes cut subjects around 33–41 characters depending on the device, so front-load the most important words. The preview here shows exactly where your subject truncates on a phone.

Which words trigger spam filters?

Words like 'free,' 'guarantee,' '100%,' 'act now,' 'urgent,' 'cash,' and excessive punctuation or ALL CAPS raise spam scores and erode trust. Modern filters weigh sender reputation more than any single word, but stacking several salesy triggers in one subject still hurts both deliverability and open rate. This tool flags them.

Do emojis help email open rates?

Sometimes — a single, on-brand emoji can lift opens for consumer audiences, but results vary widely and it can look spammy in B2B. Treat it as something to A/B test, not a default. Two or more emojis in one subject usually hurts more than it helps.