You're not "importing" an HTML file; you're sending an HTML document which uses an old experimental hack to "import" external content, giving it a relative path to an HTML file.
Aside from the fact that this experiment was removed in February 2020
], the relative path would almost certainly not exist on the recipient's computer or mail server. And if it did, and by some oversight their email client allowed the file to load, it's extremely unlikely that the file on their computer would have the same content as the file on your computer.
You need to read the contents of the file into a variable, and use that to construct your message body.
Reading files with Node.js