Above the Fold
Above the fold is the first part of an opened email that is visible before scrolling. How much fits there depends on the device and email client.
Above the fold is the first screenful of an opened email, the part a recipient sees before scrolling. The sender name, subject line and preview text shown in the inbox list are what the reader sees before opening.
How much fits above the fold depends on the device, the email client and whether images load, so there is no fixed height. A single-column layout that reads well on a phone is the usual way to keep the opening visible across clients (see responsive design).
A hypothetical example: a trial-ending email that opens with the number of days left and a “Choose a plan” button gives a scanning reader the point without scrolling. The same email with a logo banner, a greeting and two paragraphs of context before the button may push the point below the fold on a phone.
Common mistakes
Polishing layout before fixing the message. If the opening does not say what the email is about or what to do next, moving elements around rarely helps. Another is placing the only call to action inside an image, which is not shown when images are blocked.