Email Subject Line Checker
Real-time analysis of your subject line — length, spam triggers, ALL CAPS, emojis, and mobile preview. Score 0–100 with a full breakdown.
Analysis updates in real time as you type
What Makes a High-Performing Subject Line?
Your subject line is the single most important factor in whether your email gets opened. It must compete for attention in a crowded inbox, survive spam filter scrutiny, and render correctly on both desktop and mobile — all in under 60 characters.
The best subject lines are specific, curiosity-inducing, and relevant. They avoid generic promotional language, speak to a concrete benefit or question, and feel personal rather than broadcast.
Subject Line Length: Desktop vs Mobile
Desktop clients (Outlook, Gmail web) show 60–80 characters. Mobile clients — where over 50% of emails are now opened — show only 30–40 characters. The safe target is 30–50 characters.
Front-load the most important information. If your subject is "Quick question about your outbound strategy for Q3 2025", the mobile user sees only "Quick question about your outbo" — losing all context.
Spam Trigger Words to Avoid
Spam filters use word lists and contextual scoring. Individual trigger words rarely cause rejection alone, but combining several — especially with all-caps or suspicious sending patterns — pushes the spam score over the threshold.
Instead of "FREE trial", use "Try it for 14 days". Instead of "Limited time offer", use "Closing this Friday".
Frequently Asked Questions
How long should an email subject line be?
30–50 characters is ideal. Mobile clients show ~35 characters before truncating. Keep under 60 characters for full display on most devices.
Do emojis in subject lines help open rates?
One emoji can increase open rates when relevant. Multiple emojis can trigger spam filters. Test with your specific audience and ESP.
Does ALL CAPS hurt deliverability?
Yes. All-caps words are a strong spam signal. Avoid uppercase in subject lines. Title case or sentence case performs better with both filters and recipients.