I think I've found a solution, to retain double spacing and other formatting in the body of an Apple Mail message. (I wasn't able to replicate VikingOSX's results seen in the Feb 26 entry in this thread that begins "Using Word 16.58 . . ."; so this may be a complement to that approach.)
This description is what works for me with macOS 12.2.1 Monterey, MS Word v. 16.58, and Apple Mail v. 15. I can't be certain what results you'll get for other OS and app versions, but it's worth a try.
Save a formatted Word file using Save As... and select as File Format: Web Page, Filtered (.htm). There are (or on your system there may be) two Web Page formats; use the Filtered one. A prompt will ask if you want to save in this format since doing so "will remove Office-specific tags. Some Office features may not be available when you reopen this page." You'll be saving a "one-way" file that you shouldn't need to reopen in Word; to update the content of the material to be emailed, revise your Word file and write over the .htm to always have it ready to go. With the prompt quoted here, click Yes to save.
When ready to paste into an Apple mail message, open the saved HTM file, Select All, and Copy. Then Paste into the body of an Apple Mail message. This should retain double-spacing and other formatting attributes. I tested the resulting email by following VikingOSX's suggestion and sending it to a recipient who uses another email app, and that person also replied to send the same email back to me. The formatting was retained throughout that exchange.
It seems to work. I'll be happy to hear of your results in trying it!