Hey, sorry about taking a bit to get back. This sounds like it could be useful, but also a bit of a specialized feature request. We haven't had anyone else ask for this. Given that building an email template system is complicated, I think for now the best option would be copy and paste :). The rich text editor should handle you pasting from a formatted document, so you can paste from a previous email and make edits.
If we get more people requesting something like this then I don't think it would be unreasonable or us to explore this in the future. I definitely think there could be more tools for sending out emails since they are a valuable way to promote your work.