Preheader

In short: The preheader, also called preview text, is the short text that many inboxes show in the message list next to or below the subject line. It does not come from a header field of its own, but from the beginning of the email’s content.

Where the preview text comes from

For the content of an email, the Internet Message Format (RFC 5322) defines the Subject: header field, but no field for a preview text (Section 3.6.5). What an inbox shows below the subject is therefore read from the content itself – as a rule, the first words that are recognisable as text.

This has a consequence that makes many newsletters look untidy: if the very top says “Is this email not displaying correctly? Open the web version”, exactly this sentence appears in the preview. Your own preheader is therefore nothing more than a deliberately written first sentence.

Newsletters usually consist of two versions in one message: a designed HTML version and a plain-text version, packaged as multipart/alternative (RFC 2046). Which version a program shows is up to the program. The preheader therefore belongs at the beginning of both versions.

What makes a good preheader

It continues the subject line instead of repeating it. The subject names the topic, the preheader a second argument or a concrete detail.
The most important part comes first. How many characters are visible depends on the program and the screen width; there is no standard for it.
It matches the content. A preheader that announces something the email does not contain disappoints when the email is opened.

These tips are based on how the preview works, not on measured values; what works with your readers is shown by a test with two versions.

How it differs

Preheader and subject line: The subject line is a header field of the email. The preheader is part of the content. Together with the sender name, both form the first impression in the inbox.
Preheader and header: Despite the name, the preheader has nothing to do with the message header.
Preheader and teaser: A teaser in the content is meant to make people read on in an article. The preheader works before the email is even opened.

Easily confused

Web version notice: The link “View in browser” is not a preheader, but often ends up in the preview unintentionally if it is right at the top.
Invisible preview text: Some templates place the preheader as a hidden text block at the beginning of the HTML version. Whether a program shows such text in the preview or hides it completely depends on the program. A visible first sentence is the more robust solution.

Example

A club newsletter, three variants for the same mailing:

	Subject	Preheader
without its own preheader	Summer party on 21 June	Is this email not displaying correctly? Open the web version
repetition	Summer party on 21 June	Our summer party takes place on 21 June
complementary	Summer party on 21 June	Children’s flea market from 14:00, sign up by 10 June

In the HTML, the preheader is then the first paragraph, visible and short:

<body>
  <p style="font-size:14px;margin:0 0 16px;">
    Children’s flea market from 14:00, sign up by 10 June
  </p>
  …

Frequently asked questions

What is a preheader in a newsletter? The preview text that the inbox shows next to or below the subject line. It comes from the beginning of the email and can be controlled with a deliberately written first sentence.

How long should a preheader be? There is no fixed length, because every program shows a different amount. The most important part goes at the beginning; one sentence is usually enough.

Can I set the preheader in my sending platform? Some sending platforms offer a separate field for it that places the text at the beginning of the email. If not, write it as the first sentence in your template.

Matching tool

Pre-send checklist: check subject line, preview text and text version before every mailing.