What Is a Preheader in an Email? The Line That Is Not a Header

A preheader is the short line of text an inbox shows beside or below an email’s subject line. Despite the name, it is not a header: the internet message standard, RFC 5322, defines no preheader or preview field, so email platforms usually place it as hidden text at the very top of the message body.

That one fact explains three things people find confusing about preheaders: why a missing one shows “View in browser” or a logo’s alt text, why plain-text emails handle it differently, and why no one can give a universal character count. This guide reads the standard itself and three email platforms’ own help articles, all as served on October 15, 2026, and turns them into practical rules for an ecommerce brand. It comes from Relvino, a company whose product decides per shopper across email, SMS and pop-ups, so it has a stake in how brands think about email; weigh it as informed but interested.

What the standard says an email is made of

RFC 5322, the Internet Message Format published in October 2008, describes every email in two parts. In its words, “A message consists of header fields (collectively called “the header section of the message”) followed, optionally, by a body.” The header section holds named fields such as From, To, Date and Subject, each written as a name, a colon and a value. The body is everything after the first empty line, and the standard is blunt about it: “The body of a message is simply lines of US-ASCII characters.”

The header fields the standard itself names are these: trace and resent fields, the origination date, From, Sender, Reply-To, To, Cc, Bcc, Message-ID, In-Reply-To, References, Subject, Comments and Keywords. It also says “The only required header fields are the origination date field and the originator address field(s).” There is no preview field and no preheader field among them. The standard does allow optional extra fields, and the Internet Assigned Numbers Authority keeps the official registry of message header names; its Message Headers registry, last updated May 13, 2026, lists no field for preview or preheader text either, permanent or provisional.

So the subject line is a real header, carried in its own named field that every mail client knows how to read. The preheader has no such field. It is a convention built on top of the body: a mail client building the inbox list takes the subject from the header, then reads the start of the body and shows the first text it finds. Whatever text sits first in the body becomes the preview, whether the sender meant it to or not.

How email platforms put a preheader into the body

Because there is no field to fill, the three platforms read here solve the same problem the same way: it inserts the preview line as text at the top of the body and hides it from view once the email is opened. The platforms say so in their own help articles.

Mailchimp: a hidden span right after the body tag

Mailchimp’s builder has a Preview text field next to the subject line. For brands that code their own templates, its help article shows what that field actually does. A custom-coded email needs “some special CSS and HTML code that includes the *|MC_PREVIEW_TEXT|* merge tag”, and the instructions say to place a span holding that merge tag directly after the opening body tag, styled so it takes up no space and shows nothing. The article explains the styling plainly: the span tags “ensure that your preview text isn’t visible in the body of your email.” The preview text is body text that the reader of the open email never sees.

Klaviyo: preview text at the beginning of the code

Klaviyo’s help article on preview text defines it by where it comes from: it “is usually pulled from the first line of text within the body of an email.” Its Preview text field works in the drag-and-drop editor, and the same article says that for custom HTML templates “your email designer should insert your desired preview text into the beginning of the email’s code.” It also notes that the field “automatically adds spacers, so only the preview text you enter displays in your customers’ inboxes.” The spacers are invisible filler, so the inbox does not run on into the visible copy below.

ActiveCampaign: the same thing under its other name

ActiveCampaign calls the field preheader text and says the two names mean the same thing: “preheader text is a term that is often used interchangeably with preview text.” Its definition fits the same picture. Preheader text is “the first (or part of the first) line of text in your communication”, and it “will not be visible within the body of the email itself”.

• Mailchimp · What it calls the field: Preview text · Custom-coded templates, in its own words: “special CSS and HTML code that includes the *|MC_PREVIEW_TEXT|* merge tag” · Length guidance on its own page: “Most email clients limit the amount of preview text shown.” No number given
• Klaviyo · What it calls the field: Preview text · Custom-coded templates, in its own words: “insert your desired preview text into the beginning of the email’s code” · Length guidance on its own page: “keep preview text concise”; per-client limits shown only as an image
• ActiveCampaign · What it calls the field: Preheader text · Custom-coded templates, in its own words: Field available in the Custom HTML builder settings · Length guidance on its own page: “around 35-50 characters”, with “no character limit when creating preheader text”

What happens when an email has no preheader

If the preheader is just the first text in the body, then an email without a deliberate one still has a preview. It is simply whatever the template puts first. Both large platforms warn about this in nearly the same terms. Mailchimp says that without preview text, “email clients will pull in the first part of your message by default. This can include unhelpful text like a “View in browser” link or HTML code”. Klaviyo’s version is that “ALT text, your navigation bar, etc. could be pulled as the preview text”.

For an ecommerce email, that list reads like the top of a typical template. Many store emails open with a logo image, a row of navigation links (New In, Women, Men, Sale) and sometimes a shipping banner. If the logo’s alt text is the brand name and the navigation follows, the inbox preview of an abandoned cart reminder can read as the brand name followed by a menu. The fix is not a better subject line; it is putting the intended sentence first in the body, which is exactly what the preview field does.

Plain-text and text-only emails

A plain-text email has no hidden markup, so there is nowhere to hide a preview line. The platforms handle that limit differently, and the difference matters for brands that send founder-style plain emails or text-only win-back notes.

• Klaviyo uses the opening of the email itself: “For text-only emails, the very beginning of your email is used as preview text.”
• ActiveCampaign turns the feature off: “Preheader text is not available with 1:1 emails or plain-text campaigns”.

Both answers follow from the same fact. With no header field and no hidden text, the preview can only be the first words the reader will also see when the email is opened. In a plain email, the first sentence does two jobs at once: it is the inbox preview and the opening line. Write it for both, and do not open with a greeting that wastes the preview on “Hi there,”.

How long should a preheader be?

There is no standard answer, and the reason is structural. The standard defines no preview field, so it sets no length for one, and each mail client decides for itself how much body text to show in the inbox. The three platforms reflect that in their guidance:

• ActiveCampaign is the only one of the three that gives a number. It says “there is no character limit when creating preheader text”, then recommends keeping it short, “around 35-50 characters”. It lists what decides how much a reader sees: the device, the email app, the browser window width on a desktop and the length of the subject line.
• Klaviyo says “Different devices and email clients have different limits on the amount of preview text characters displayed, with smaller screens displaying fewer preview characters”, and asks senders to keep preview text concise. Its per-client limits appear only in an image it credits to Email on Acid, so they are not reproduced here.
• Mailchimp gives the advice without a figure: “Keep it short”, because “Most email clients limit the amount of preview text shown.”

The practical reading: any character count found online is one vendor’s recommendation or one test of particular clients, not a rule. ActiveCampaign’s point about subject length is the useful one for a store. The subject and preview share the same inbox row on many screens, so a long subject leaves less room for the preview, and the preview’s first few words are the only ones a brand can count on being seen. Put the specific thing (the product, the offer, the deadline) at the front of the line.

What a preheader is for

The platforms agree on the job. Klaviyo names two uses: to “summarize the email” or to “complement your subject line”. Mailchimp’s best-practice list says to “Start with the most important details” and to “Support the subject”, adding that if the important information does not fit in the subject line, “now is your time to expand on it.” Neither says to repeat the subject, and repeating it wastes the only other line the shopper reads before deciding.

Vendors also sell the preheader as an open-rate lever; ActiveCampaign’s article says it can help increase open rate. Treat that claim with care, because opens are a weak measure now. The email marketing KPIs guide explains why, and it is the better place to decide what to read after a send.

Preheader examples for ecommerce emails

A good preheader adds the one fact the subject line left out. The examples below are illustrations written for this guide, not vendor templates or tested winners; each pairs a subject with a preview line that does a different job.

• Abandoned cart. Subject: “Still thinking about the trail jacket?” Preheader: “Your size is in stock, and returns are free for 30 days.” The subject names the product; the preview answers the likely hesitation. The abandoned cart subject lines guide covers the subject half of the pair.
• Welcome. Subject: “Welcome to the club.” Preheader: “Here is the code for your first order, plus what to expect from us.” The preview tells a new subscriber why opening is worth it. More on the first message in welcome email examples.
• Shipping confirmation. Subject: “Your order is on its way.” Preheader: “Arriving Thursday. Tracking link inside.” Here the preview is pure information, which suits a message whose purpose is the order itself; the line between that kind of email and a promotional one is drawn in transactional vs marketing email.
• Win-back. Subject: “It has been a while.” Preheader: “New colors in the tote you bought last spring.” The preview makes the message about the shopper’s own history rather than a generic nudge.

Two habits follow. First, check the preview of every template, not just every campaign, because a template that starts with an image or a menu leaks it into every send built on it. Second, when a brand codes its own templates, make sure the preview line really is the first text in the body; a hidden tracking element or a stray text block placed above it will take its place in some inboxes.

Where Relvino fits

Relvino decides for each shopper whether to send an email, a text or a pop-up, or nothing, and what offer, timing and creative to use, in under 80ms per decision, inside guardrails the brand sets. Its Large Retail Model is trained on 7M+ data points across 10K+ retailers, so a store gets category knowledge before its own history is deep. An email Relvino sends is still a message with a header section and a body, and its preview line is still the first text an inbox finds, so the same rules about what sits at the top of the body apply to it as to any other sender.

A header in name only

The word preheader suggests something above or inside the header, and both readings are wrong. The subject line is the header the inbox reads; the preheader is the first thing in the body, dressed up by each of these platforms to behave like a second subject line. Once that is clear, the practical questions answer themselves. A missing preheader shows the template’s first text because that is all it ever was. Plain-text emails cannot hide one because there is no markup to hide it in. And no vendor can promise a length, because the only party that decides how much of the body to show is each reader’s mail client.

Frequently asked questions

What are the best practices for email preview text?

Beyond keeping it short and leading with the key detail, a few platform-specific notes help. Klaviyo’s help article warns that Gmail may remove commas inside numbers in the preview, so 10,000 can show as 10000; write prices and counts with that in mind. ActiveCampaign lets senders add personalization tags and emoji to preheader text and offers a mobile preview of how the line will look, which is worth checking before every send because mobile screens show the fewest characters.

What are the 5 main components of an email?

The message standard, RFC 5322, names only two parts: a header section and an optional body. Marketers usually break a sent email into five working pieces instead: the sender name and address, the subject line, the preheader, the body with its call to action, and the footer that carries the unsubscribe link and postal address. The first two live in header fields; the last three all live in the body, preheader included.

Is preheader text the same as preview text?

Yes. ActiveCampaign’s help center says the two terms are often used interchangeably, and Mailchimp and Klaviyo both label the field Preview text. Some people use preheader for the text a sender writes and preview for what the inbox ends up showing, but no platform among the three draws that line. Klaviyo also offers a tag that prints the saved preview text inside the visible email body, for brands that want the line to appear in both places.

Does Outlook show preheader text?

Not always. ActiveCampaign’s help center says preheader text may not always be supported, giving some Outlook clients as its example. Mailchimp’s code for custom-coded templates wraps its hidden preview span in a conditional comment aimed at Outlook, which its article says helps the preview text display properly there. Brands with many Outlook readers, such as those selling to businesses, should test the preview in Outlook specifically.

Can preheader text be hidden or left blank?

It can be suppressed, but not removed outright, because any first text in the body becomes the preview. Klaviyo’s help article gives a snippet to place in an HTML block at the very top of an email that fills the preview area with invisible spacer characters, pushing the real content past it so only the subject line appears to show. Leaving the field empty without that trick does not produce a blank; it lets the template’s first text through.

How long does it take to migrate email to Relvino?

About 30 minutes for the technical cutover: connect the Shopify store, point the sending domain, and shopper data ingests automatically. Revenue is then measured in a 14-day pilot beside the existing setup, so a brand can compare results before changing its current plan. There are no flows or templates to rebuild first, since Relvino decides each message per shopper rather than running a prebuilt sequence.