Every email in an inbox list gets three pieces of information: who it is from, the subject line, and a preview snippet. Teams routinely rewrite the subject line a dozen times and let the third one be chosen at random by the mail client.
What the client is doing
Mail clients build the preview by taking the first readable text in the message body. There is no way to opt out — if you do not provide something suitable first, it takes whatever is there.
Which is why inboxes are full of previews reading "View this email in your browser", "Email not displaying correctly?", or the alt text of a logo image. The sender did not choose those. They simply appeared first in the HTML.
Setting it deliberately
The standard technique is a hidden block at the very top of the body, before any visible content:
<div style="display:none;max-height:0;overflow:hidden;
mso-hide:all;font-size:1px;line-height:1px;color:transparent;">
Your preheader text goes here.
</div>It is invisible when the message is opened and is the first text the client finds, so it becomes the preview.
One refinement worth knowing: some clients will continue pulling text after your preheader ends, appending the next visible content. Adding a run of zero-width characters (‌ repeated) after the preheader block absorbs that remaining space and stops "Hi {'{'}FirstName{'}'}" being stitched onto the end of your carefully written line.
Writing one worth reading
The mistake is repeating the subject line. The two appear together, so a repeat wastes the second slot entirely.
Think of them as one sentence in two parts — the subject raises something, the preheader gives the reason to open.
| Subject | Weak preheader | Better preheader |
|---|---|---|
| Your DMARC report is ready | DMARC report ready to view | Three senders failed authentication this week |
| New sending limits explained | Read about the new limits | What changes for accounts sending over 5,000 a day |
| Quick question about your setup | A question about your setup | Specifically, how many domains you send from |
Practical rules
- 40 to 90 characters. Front-load, because mobile truncates earlier than desktop.
- Never repeat the subject. Two slots, two jobs.
- Avoid trigger language. It is scanned like any other content — our spam score checker covers the phrases worth avoiding.
- Personalise carefully. Merge fields work here, but a failed merge showing a raw placeholder in the inbox preview is a conspicuous way to look automated.
- Check what actually renders. Send yourself a test and look at the inbox list, not the opened message. The preview is the only part you cannot see from inside the email.
Why it is worth the five minutes
Preheader text will not rescue a campaign that has deliverability problems — if the message is in the spam folder, nobody sees any of the three fields. But for mail that does reach the inbox, it is a free line of persuasion that most senders leave to chance, and improving open rates feeds engagement, which feeds reputation.
It is one of very few things in email marketing that costs nothing, carries no risk, and most competitors are not bothering with.