What makes a good email subject line?
- Short and front-loaded. Phones show about 35–45 characters. Put the most important words first.
- Specific. “Your order #4821 has shipped” beats “An update for you”.
- Honest. The subject must match the email. Misleading subject lines are banned by the US CAN-SPAM Act and annoy readers everywhere.
- Low on hype. Words like “FREE!!!”, “act now” and “guaranteed”, ALL CAPS and lots of exclamation marks can trigger spam filters.
- Use the preview text. The preheader is a second line of free space. Do not leave it as “View this email in your browser”.
| Weak | Better | Why |
|---|---|---|
| FREE SHIPPING!!! ACT NOW | Free shipping ends tonight at midnight | No caps or hype, with a clear deadline |
| Newsletter #47 | 5 budget dinners under $10 this week | Specific benefit and a number |
| Hi | Quick question about your Q4 order | Gives context so it isn’t ignored |
| You won’t believe this | Our new app saves 2 hours a week | Honest claim, no clickbait |
Do spam words really matter?
Modern filters at Gmail and Outlook care most about your sender reputation, authentication (SPF, DKIM, DMARC), and whether people open or delete your mail. A single “free” will not send you to spam. But a subject packed with sales phrases, caps and symbols makes filters suspicious and looks untrustworthy. Read more in our spam trigger words guide, or check your whole email with the spam word checker.
Need ideas? See Black Friday subject lines, Christmas subject lines and Halloween subject lines.
Email Subject Line Tester FAQs
How long should an email subject line be?
Aim for about 30–50 characters, or 4 to 9 words. Phones cut most subject lines at around 35–45 characters, so keep the key words at the start.
Do emojis help open rates?
One relevant emoji can help a subject stand out, especially for seasonal emails. More than two often looks spammy, and some older email apps show them as boxes.
What words trigger spam filters?
Phrases like “act now”, “100% free”, “guaranteed”, “cash”, “no credit check” and “you’re a winner” are classic triggers, especially combined with ALL CAPS and exclamation marks.
Is the score the same as my open rate?
No. The score checks best practices. Your real open rate depends on your list, sender reputation and timing. A/B test two subject lines when you can.