in5 produces an index.html file.
I do not recommend trying to use this to paste into an email. The HTML inside of email clients is outdated and does not support modern layout capabilities.
More info: What’s the Difference Between an HTML5 Email and a Unicorn?
Additional details in this answer as well: https://ajarproductions.com/pages/products/in5/answers/1955/in5-to-mailchimp-html