Line spacing in Mac mail

There are no line-spacing controls in the Apple Mail compose window as it is not a word-processing tool.

If you have MS Word and write your Mail message in it with appropriate line spacing; copy to and paste from the clipboard into the Mail compose window, line-spacing is preserved in that compose window, and upon sending, also in a recipient's Apple Mail. There is no guarantee that this specific line spacing will be preserved in other Mail clients, especially if they are using plain and not rich text.

Apple's Pages does not include the extra styling baggage of MS Word content and its line spacing is not preserved when pasted into Apple Mail.