How to test an email subject line
- Enter the sender name, subject line and preview text (also called preheader).
- See how the email will look in a phone inbox and a desktop inbox, including where the subject gets cut off.
- Review the score and checks: length, spam-trigger phrases, capital letters, exclamation marks, emoji, fake reply prefixes and missing preview text.
- Rewrite until the score is green, then copy your subject into your email platform.
Writing subject lines that get opened
About half of emails are opened on phones, where only around 35–45 characters of the subject are visible. Put the most important words first and keep the whole subject between roughly 30 and 50 characters. Use the preview text as a second line that adds detail, rather than repeating the subject; if you leave it empty, inboxes show the first text in your email, which is often "View this email in your browser".
Specific beats clever: "Your March invoice is ready" or "3 recipes for busy weeknights" tell readers exactly what they get. Questions, numbers and genuine urgency (a real deadline) tend to lift open rates. Personalisation helps when it is relevant, not just a first name.
Avoiding the spam folder
Modern spam filters rely mostly on sender reputation, authentication (SPF, DKIM and DMARC) and how recipients engage, but spammy wording still hurts both filtering and trust. Avoid phrases like "100% free", "act now" and "you have been selected", words in capitals, piles of exclamation marks and fake "Re:" prefixes. The list here covers common triggers; it is a guide, not a guarantee.
Everything is checked in your browser; nothing is sent anywhere.
What is the best email subject line length?
About 30–50 characters or 4–9 words. Shorter subjects show in full on phones; longer ones get cut off.
Should I use emoji in subject lines?
One relevant emoji can help a subject stand out, especially in consumer email. Avoid several, and test how they render in Outlook.
What is preview text?
The short line shown after the subject in most inboxes. Set it with your email platform's preheader field or a hidden first line in the email.