What is an inbox preview?
An inbox preview is how a message appears in the list before it is opened: the sender name, the subject and a line of preview text taken from the start of the body, which is where a preheader goes. Lists cut each of these where it runs out of room, so the first words carry most of the weight.
Reading the result
Each field shows its length in characters, counted as a reader sees them, so an emoji or an accented letter counts once. The preview lays the text out in two row shapes, one line and stacked, at the width you pick, and says how many characters of each field fit before the cut. It is a drawing in one stated font, not a copy of any mail client.
What it helps you catch
Trusting a character limit
Inbox lists cut by width, not by count: the same number of characters takes more room in capitals or wide letters than in narrow ones, and each client, font and window differs. Put the point of the subject in its first few words instead of aiming for a number.
A preheader that repeats the subject
If the preview text starts with the subject again, the reader sees the same words twice and learns nothing new from the second line.
No preheader at all
The preview line is built from the body, so without a preheader it shows whatever comes first, such as a "view in browser" link if that opens the message.
Preview text the reader has turned off
Outlook lets readers hide preview text in the message list, so the subject has to work on its own.