What I personally do is really simply.
I create an actual populated HTML document in Dreamweaver and save it into the assets folder. If necessary I embed keywords like #Name, #Address, #ZipCode, #Telephone etc that can be replaced with an actual name, Address, ZipCode, Telephone Number when reading into SMTPClient.Body. I then read the HTML file into SMTPClient.Body replacing #Name, #Address etc if I have including them in the HTML file.
That's it, it's easy to do and you do not have to worry about double quotes and guess work in your actual B4A code. Plus you can get the HTML side of things looking really nice and fancy before loading the html file into the SMTPClient.Body.
Enjoy...