You spent an hour on the subject line, and the grey line under it says "View this email in your browser". That line is the email preheader, also called preview text, and it's the second thing people read in the inbox after your name and the subject. Write it on purpose and it earns the open by adding something the subject didn't say. Leave it empty and the inbox fills the space with whatever comes first in the email. In Milo Email Studio, the preview text sits next to the subject in the editor, so you write them as a pair, and the AI designer can suggest both if you're stuck.
What a preheader is
A preheader is the short snippet an inbox shows next to or under the subject line. Preheader, preview text and preview line all mean the same thing. It comes from the email itself, so you write it once and every client that shows previews pulls it from the file.
It isn't the subject, and it isn't the summary Apple or Google may write for some readers. It also has nothing to do with a preview link you share for approvals.
In the HTML it usually sits as a hidden line at the very top of the body, so it shows in the inbox but not in the open email. In MJML it's the mj-preview tag, which "allows you to set the preview text that will be displayed in the inbox of the recipient".
Make it the second subject line
The subject makes the promise. The preview line adds the detail, the deadline or the reason to care. When the two repeat each other, you've spent your best inbox space saying one thing twice.
These pairs are made-up examples to show the difference:
| Subject | Preview that echoes | Preview that adds |
|---|---|---|
| Our autumn sale starts today | Shop the autumn sale now | 20% off coats and boots until Sunday at midnight |
| The October edition | Read our October newsletter | Why we changed roasters, plus three new recipes |
| Welcome to Hollow Oak | Thanks for joining Hollow Oak | Your first order ships free. Here's what to try first |
Keep the preview in the same voice as the email. If your emails are warm and plain, a shouty preview line feels like a different sender.
What the inbox shows when you leave it empty
With no preview text set, the client shows the first text it finds in the email. That's often "View in browser", a logo's alt text, the first link in a menu or a line of legal small print. None of it helps anyone decide to open.
The fix is simply to write one. The HTML email checklist covers the preview line alongside the other checks before you export, and it's worth running through the whole list.
A hidden preview line also sits right at the top of the file, so even when Gmail clips a long email, the preview is safely before the cut. If clipping is the worry, see how to stop Gmail clipping your email.
How long should preview text be?
There's no single right length, because each client shows a different amount. On iPhone, Apple Mail shows two lines per message by default, and readers can choose up to five. Classic Outlook's Message Preview shows between one and three lines, or none at all. Screen size and the reader's own settings change it again.
So put the point in the first few words and treat everything after that as a bonus. Testing tools publish rough guides if you want one, but no client documents a fixed character limit.
If your preview is short, some clients may carry on with the next text in the email to fill the line. Read it in a real inbox to see what follows.
Keep it on brand and honest
The preview line should match what the email delivers. If it promises a discount, the discount should be in the email. Gmail's sender guidelines ask that message content isn't misleading, and a preview that's hidden in the code should still say the same thing as the email people open.
You can personalise it with your ESP's merge tags, such as a first name. Make sure the line still reads well when that field is empty.
When AI summaries take the line
Apple Mail with Apple Intelligence and the Gmail app with Gemini can show a short summary in place of the preview for some readers, on some devices, with the right settings on. Plenty of readers still see your preview text, so write a real one anyway.
Designing the body so those summaries have something accurate to work with is its own topic. It's covered in designing emails for Apple Mail and Gmail AI summaries.
Check it in a real inbox
You only see the subject and preview as a pair once they land in an inbox. Send yourself a Test from Milo to read the email on your phone and on your desktop. A Test's subject starts with "Test", so also check the real subject and preview line together in your ESP's own test or preview before the campaign goes out.
Write the subject and preview together in Milo
Describe the email, and the AI designer drafts the on-brand blocks along with a subject and a preview line written to go together. Edit either one, or the blocks, on the canvas. The preview text sits in the email's settings next to its other details, with a character count as you type, so you can see how long it's getting.
When you're happy, export-ready HTML carries your preview text as a hidden line at the top of the email, and MJML export carries it as mj-preview. More on choosing between them in HTML vs MJML export. If you use Zoho Campaigns, connect it and push a draft, and the preview text is already in the email. For other ESPs, export the file and import it, then glance at the preview line in the ESP in case its editor has a preview field of its own.
Questions
What is an email preheader?
The short line of preview text an inbox shows next to or under the subject line. It comes from the email itself, usually as a hidden line at the top.
Is preheader text the same as preview text?
Yes. Different tools use different names for the same line.
How long should a preheader be?
There's no single number. Clients show different amounts depending on the device and the reader's settings, so put the point in the first few words.
What happens if I leave it empty?
The inbox shows the first text it finds in the email, which is often "View in browser", a link or alt text.
Should the preheader repeat the subject?
No. Use it to add something the subject didn't say.
Do Apple Intelligence and Gmail replace preview text?
Sometimes. Apple Mail can show a summary instead when the setting is on, and Gmail's Gemini can replace the preview on some messages for eligible accounts. More in designing for AI inbox summaries.
Can I personalise it?
Yes, with your ESP's merge tags. Make sure it still reads well if the field is empty.
Can Milo's AI designer write it?
Yes. It drafts the subject and preview text along with the blocks, and you can edit both.
Is the preview text in my export?
Yes. Export-ready HTML has it as a hidden line at the top, MJML has it as mj-preview, and a Zoho Campaigns draft carries the same HTML.
Where does the campaign go out from?
From your ESP, where you choose the list and the send time.