Above the fold
The part of an email visible before the reader scrolls.
The phrase comes from newspapers, where only the top half of the front page showed on the stand. In email it means whatever fits in the preview pane or on the first screen of a phone before scrolling.
There is no fixed height. It depends on the client, the window and whether a reading pane is open, so the useful question is not “where is the fold” but “does the first screen make sense on its own”.
In practice: put the point and the main action near the top, and do not spend the first screen on a logo and a hero image alone.