With v5.4, TMail has more ad spaces.
Below is a short preview of how many and where exactly are the Ad Spaces are located in TMail:
If you still wish to add more ads, you will need to edit following files based on where you want to add ads:
1. resources/views/layout.blade.php - Global Layout
2. resources/views/page.blade.php - Page Layout
3. resources/views/app.blade.php - App Layout
You will need to have basic knowledge of HTML / PHP to understand how to add the ads code.
However, at every update, you may need to do this again since you're editing the core files.