Email subject line checker
Paste your subject line and get an instant inbox-friendliness score. We check for spam-trigger words, length, ALL-CAPS, punctuation, and emoji — with tips to help you land in the inbox.
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A great subject line still needs to reach the inbox
Wording is one factor — sender reputation, warm-up, and authentication do the rest. AetherMAIL is deliverability-first cold email software built to land.
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What makes an email subject line "spammy"?
Spam filters don't judge a subject line on any single word. They weigh dozens of signals together — and subject-line wording is a small but visible one. The patterns that raise risk are the ones that look like mass marketing: shouting in ALL CAPS, stacking exclamation marks, leading with money and urgency ("FREE!!! Act now"), or burying the real message under emoji and symbols. This checker looks for those patterns and gives you a plain-English score so you can rephrase before you send.
How the score works
Your subject line starts at 100 and loses points for each risk signal we detect:
- Spam-trigger words — common filter triggers like free, guarantee, act now, winner, risk-free, cash, limited time.
- Length — the sweet spot is roughly 30–50 characters; very long lines get truncated on mobile.
- ALL CAPS — shouting reads as promotional and hurts trust.
- Excessive punctuation — multiple ! or ?, or symbols like $$$.
- Emoji overload — one can be fine; several signal a marketing blast.
The result is guidance, not a verdict. Use it to catch obvious mistakes — then let your sender reputation and relevance do the heavy lifting.
Tips for subject lines that get opened
- Be specific and human — write like one person emailing another, not a campaign.
- Lead with relevance — reference the recipient's world, not your offer.
- Keep it short — 3–7 words often outperforms a full sentence.
- Skip the hype — no ALL CAPS, no "FREE", no triple exclamation marks.
- Test, don't guess — try variants and watch reply rate, not just opens.
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Subject line checker — questions
What words trigger spam filters in email subject lines?
Classic triggers include free, guarantee, act now, urgent, winner, cash, risk-free, limited time, and money-back — especially combined with ALL CAPS, multiple exclamation marks, or currency symbols. No single word always sends you to spam, but stacking several raises risk. This tool flags common triggers so you can rephrase.
How long should an email subject line be?
Aim for roughly 30–50 characters. Shorter lines are easy to scan and display fully on mobile, where much email is opened. Very long subject lines get truncated and can look like marketing blasts.
Does this checker guarantee inbox placement?
No. Subject-line wording is only one factor. Deliverability depends mostly on sender reputation, authentication (SPF, DKIM, DMARC), list quality, and sending behaviour. This tool helps you avoid obvious subject-line mistakes; it can't guarantee the inbox.
Is this tool really free?
Yes — it runs entirely in your browser, needs no signup, and nothing you type is sent to a server or stored.