To clarify this - making a custom post template inside the theme does not affect the appearance of what goes out by email. It only affects how it looks on the website.
There is modest ability to customize the newsletter that gets emailed out, and you’ll find it inside /ghost > settings (the gear icon), under newsletters:
(when you hover, an ‘edit’ link appears for each newsletter)
Then within the Design tab (see image below), there are some options you can select, like whether to show the post excerpt, whether to show the blog/newsletter title or the post title, etc.
More changes would require editing the core of Ghost, which would mean needing to merge code whenever Ghost releases an update (i.e. weekly, if you want to stay up to date). There’s some discussion of how to do so here:
(The start is likely outdated, but the last post should get you to the right part of the file structure, I think.)