# Email subject line and preview text checker

> Paste a subject line and preview text to see how an inbox shows them and get instant checks on length, capitals, punctuation and misleading prefixes; it runs in your browser and needs no email address.

Canonical: https://tellnod.com/tools/subject-line-checker  
Last updated: 2026-10-15  
Author: Tellnod team

This is an interactive tool. It runs in the browser at the page URL and sends nothing to Tellnod.

## What it checks

- Length against what phones and desktop inboxes usually show
- Mostly capital letters and repeated punctuation
- Misleading "Re:" or "Fwd:" prefixes (CAN-SPAM bans deceptive subjects) [S2]
- Merge tags that need a fallback
- Missing, short, long or repeated preview text

## FAQ

### How long should an email subject line be?

Short enough that the key words show on a phone: phones often cut after about 40 characters, so put the important words first and keep it under about 60.

### What is preview text?

The line an inbox shows next to or under the subject. If you leave it empty, the inbox shows the opening words of the email. See [preheader](https://tellnod.com/glossary#preheader).

### Do you store what I type?

No. The checker runs entirely in your browser; nothing is sent to us.

## Sources

- [S2] FTC: CAN-SPAM Act compliance guide for business: <https://www.ftc.gov/business-guidance/resources/can-spam-act-compliance-guide-business> (checked 2026-10-15)