What makes a good email subject line
Your subject line is the single biggest driver of whether an email gets opened — more than the sender name, more than the send time. A few principles hold up across almost every audience:
- Keep it short. Most email is opened on a phone, where roughly the first 33 characters show. Aim under 40; up to ~60 is usually safe on desktop.
- Front-load the point. Put the compelling words first, so the line still works if it gets truncated.
- One clear idea. Curiosity, a benefit, or a genuine reason to open — not three at once.
- Go easy on spam triggers. "Free", "guarantee", "act now" and friends — especially with ALL CAPS and multiple exclamation marks — raise your spam risk.
- Emoji: one, maybe. A single well-placed emoji can help it stand out; two or more often reads as spam and renders unpredictably.
How to use this tester
Type or paste a subject line above. As you type, the tester scores it out of 100 and flags anything worth fixing — length against mobile and desktop cut-offs, spam-trigger words, shouty capitals, and emoji. The inbox preview shows roughly how it lands. Tweak until the score is green, then send with confidence.
Frequently asked questions
How long should an email subject line be?
Under about 40 characters keeps it safe on mobile, where most opens happen. Up to ~60 is usually fine on desktop. Front-load the important words in case it truncates.
Do emojis in subject lines help?
One well-chosen emoji can lift opens; two or more often looks spammy and renders inconsistently across clients. Always test before relying on it.
What words trigger spam filters?
Classics include free, guarantee, act now, buy now, winner, cash and 100% — worse when combined with capitals and exclamation marks. Modern filters weigh many signals, so treat these as warnings, not hard rules.
Want every check in one place before you send? Try the full Mailfois pre-send checker — subject, preheader, links, deliverability, accessibility and inbox preview in one readiness score.