First thing first, if you're talking about a neatly - or in any way - styled email, this is not possible. The specifications of mailto, which is not related to Odoo in any way, prevents you from doing this (see for example a discussion here: https://stackoverflow.com/questions/5620324/mailto-link-with-html-body).

If that's not an issue, you can define a mailto's body like this:

<a href="mailto:mail@example.com?subject=Hello world&body=Line one%0DLine two">Mailto Link</a>

Now, how to implement this, depends on how/where you'd like to use it. In any case it requires custom development.

Apart from that, you can always create mail templates in Odoo for later use. You can however not create a draft (like you know it from a mail client of your choice) for a mail in Odoo (again, without customization, that is).