How can I change line spacing in Mail?

Solution 1:

Apple's html editor doesn't engage in that level of formatting. It's still a throw back when mail was plain text and designed to let the receiver decide how to render line breaks and spacing as opposed to being a word processor.

• Some love this since it's optimized for simplicity, speed and clarity.
• Others despise it and want a heavy editing / formatting quiver at their disposal. (or at least heavier in function than was shipped)

You'd need a third party editor (or other app) to pre-author things and compose the mails or choose another client that suits your requirement to get line spacing rich text / html.