I would like to use the macOS mail app to compose mails with non-standard line spacing.
Are there solutions without using 3rd party software?
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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.
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.