Preview text is your second subject line.
Yours just repeats the first one.
A client sent me their campaign the morning it went out, proud of the subject line. It read "Your June restock is live." Good enough. Then I opened the inbox preview on my phone and saw the line underneath it: "Your June restock is live." Word for word. The subject, then the subject again, taking up the most valuable second line the inbox gives you and saying nothing new. The open rate came in flat, a little under their usual, and we both knew the headline did all the work alone while the line beside it just sat there repeating itself like an echo.
That line has a name. It's the preview text, sometimes called the preheader: the snippet the inbox shows next to or beneath the subject before anyone opens. Gmail, Apple Mail, and Outlook all pull it into the list view. It's the second thing a reader sees, every time, in every client. And in my audits I often find it doing one of two useless things: repeating the subject, or auto-filling with whatever copy happens to sit first in the email body. Both are non-decisions, and each one wastes a slot you get for free.
Where the bad preview text comes from
If you don't set preview text, the inbox doesn't leave it blank. It grabs the first readable text in your HTML and shows that instead, and usually that's a "View in browser" link, an address block, or the opening line of a header that was never written to be read in a list view. So the reader sees "Trouble viewing this email? Click here" sitting under your subject line, and that's the first impression your campaign makes. The fix people reach for is to repeat the subject in the preheader so at least it isn't garbage, but that just trades garbage for redundancy. You still spent the slot saying one thing twice.
Treat it as a second subject line
The better frame is the one in the title: preview text is a second subject line, and the two should never say the same thing. The subject carries the hook, and the preview text continues it. They're a setup and a follow, not a statement and its echo.
The simplest rule that works: the preview text has to add one new piece of information the subject doesn't contain. A name, a number, a deadline, a benefit, an objection answered. If your subject is "Your June restock is live," the preview can be "Three pieces from the waitlist sold out in April. They're back." Now the two lines do two jobs, the subject pulls the eye and the preview gives a reason to act. The reader learns more in the list view than they did before, which is the entire point of the line existing.
A few specifics from doing this across a lot of campaigns:
Continue, don't summarize. The preview isn't a recap of the email, it's the next sentence after the subject. Write them together, out loud, as one thought broken across two lines.
Front-load the new information. Inboxes truncate the preview, and the cutoff length varies by client and device, so anything you care about has to land in the first stretch. Put the new fact at the start, not after a wind-up.
Keep it tight. There's no single safe length across clients, so write the line to read complete in roughly the first sentence and treat anything past that as a bonus.
Don't stuff keywords. A preview crammed with the same three offer words reads like spam to a human and adds nothing the subject didn't already say.
Hide the rest so it does not leak in
There's one technical step that separates a controlled preview from a sloppy one. Even when you write a deliberate preview line, stray body copy can leak in after it. The inbox shows your preheader, runs out of your words, and then keeps pulling from the next readable text in the HTML, so the reader sees your clean line followed by a fragment of a headline or a button label.
The way to stop it is a hidden preheader: a div at the very top of the body that holds your preview text and is styled so a human opening the email never sees it, while the inbox still reads it for the list view. You set it to display none, then follow it with a run of space characters like  ͏. That filler eats the remaining preview window, so the inbox runs out of room on your invisible spacer instead of grabbing your "View in browser" link, and the line you wrote is the only thing it shows. It's a small block of markup you write once and reuse on every template, and it's the difference between a preview you control and one the client assembles for you.
The whole thing costs you one sentence and a few lines of HTML. For that you get a second subject line on every send, working alongside the first instead of repeating it. The client whose restock email echoed itself writes the two lines together now, and the second line earns its place in the inbox the same way the first one does.