Preheader text is the short line of copy shown next to or below the subject line in an inbox, giving readers a preview of the email before they open it. If you do not set it, most mail apps pull in the first text they find, which is often “View this email in your browser” or a line of image alt text.
How preheader text works
There are two ways to set it. Most email platforms have a “preview text” field: you type the line, and the platform places it at the top of the email’s code, hidden from view once the email is opened. Alternatively, it can simply be the first visible line of the email, which then appears at the top of the message as well.
How much of it appears depends on the mail app, the device, and the length of the subject line and sender name. A phone in portrait shows less than a desktop inbox, and a long subject line pushes the preheader out of sight. As a working rule, put the important words in roughly the first 40 to 50 characters and treat anything after that as a bonus. That is a practical guide rather than a fixed limit. Some senders add invisible spacing characters after the preheader so body text does not spill into the preview.
You may see it called a Johnson box in older guides. The name comes from printed direct mail, where a boxed summary sat above the start of a sales letter.
Why it matters
The subject line, sender name and preheader are everything someone sees before deciding whether to open, ignore or delete. A preheader that adds information, such as a date, a price in pounds or what is inside, gives a second reason to open. One that says “Having trouble viewing this email?” wastes the space and makes the message look automated.
It also affects how trustworthy an email feels. A preheader that contradicts the subject line, or promises something the email does not deliver, reads as a trick, and readers who feel tricked unsubscribe or report spam. For promotional messages, the CAP Code covers the whole communication, so a claim made in the preview must be one the email itself can back up.
Common mistakes
- Leaving it blank, so the preview shows “View in browser”, a legal notice or alt text.
- Repeating the subject line word for word.
- Putting the key point at the end, where phones cut it off.
- Forgetting to update it when a template is reused, so the preview mentions last month’s offer.
- Using a personalisation token without a fallback, so the preview shows a blank or a code.
How to act on it
Write the subject line and preheader together, as a pair. Let the subject line make the main point and the preheader add the next most useful detail. For a fitness studio, that might be “Spring timetable is live” followed by “New Saturday morning classes start in May, and booking is open now”. Before sending, check the inbox preview on a phone and a desktop, using your platform’s preview tool or a test send to two or three different mail apps.
Add a preview text check to your sending checklist so it is never left on the default. Reviewing email templates and sending routines like this is part of my digital marketing strategy and consulting work.