How a preheader works
When an inbox lists an unopened email it shows the sender, the subject line and a short piece of text pulled from the message itself. That third element is the preheader, and unless you set it, the app simply takes whatever text appears first in the email body.
That default is why so many campaigns preview with something useless. If the top of the template carries a view-in-browser link, an alt text label or an unsubscribe note, that is what the reader sees in the inbox. Most email platforms provide a field to set the preview text explicitly, and some templates use a block of hidden text at the very top of the message to do the same job.
How much of it appears varies. Some apps show a generous stretch, some show a few words, some show none at all and give the subject line the whole row. The email must therefore work whether the preheader is displayed in full, cut short, or never seen.
Why the preheader matters
It is free space beside the only other line the reader is looking at. A subject line has to be short, so a good deal has to be left out of it — the preheader is where that missing detail goes. The two read together as one sentence, and setting them as a pair is a genuinely different exercise from writing each alone.
It also removes a small, avoidable signal of carelessness. An inbox full of your emails all previewing with the same view-in-browser text tells a reader that nobody checked, and that impression carries over to whatever the email is asking them to do.
Where preheaders go wrong
Leaving the field blank is the most common problem, and it is invisible from inside the email platform — everything looks correct in the builder while the inbox shows something else entirely.
Repeating the subject line word for word is the next. It doubles one message instead of adding a second, wasting the space. Some senders go the other way and pad the preheader with invisible characters or blank space so that nothing shows at all, which achieves nothing and can look odd in apps that render it. Writing it as a separate sales line, disconnected from the subject, has the same effect as an advertisement with two competing headlines.
How to write one
Draft the subject and preheader together, as a pair, and read them the way an inbox prints them: sender, subject, preview. If the second line does not add something the first left out, rewrite it.
Put the useful words first, because the tail may be cut. Keep it plain — a date, a deadline, what is inside, who it is for — rather than another attempt at persuasion. Then send a test to yourself and look at it on a phone and on a desktop client, since how a message is displayed differs enough between apps that the builder’s preview cannot be trusted on its own.