I am working on a solution that has same requirements you posted, except that I use docx format.
Could you please let me know (provide sample code etc..) how you solved the below points programmatically.
1. In what format you saved word to html (mht or html with _files folder where the images/headers are stored)
2. how did you saved the html letter in to the body of outlook
3. Does it display content + images/logos
Any help would be greatly appreciated,