Supersript in Mail?

Provided that your superscript character is numeric, or limited to a small subset of other characters, place the insertion point where you want the superscript, and then press control+command+spacebar. This will launch the Emoji & Symbols panel (character viewer), where you can enter the word superscript (or alternatively, subscript) in its search field. This will fill the center area of the viewer with a selection of characters and when you click on them, it will report if that character is a super/subscript.

You then double-click on the super/subscript character that you want inserted into your Mail compose location.

I think you have to compose it in another App that supports superscripts, then to ensure the other end sees it as superscript you'll save it as PDF & attach that.

My eMail is set to text only, no Rich Text/RTFs.