From the pop-up menu, choose Font > Styles…. Select the text that you want to apply a style too. Now, Superscript, Subscript, and Strike-through are now selectable Favorites on your Show Styles menu. Choose Add to Favorites, and when the naming sheet slides down, call it Strike-through, or Red Strike-through (if pertinent). Select this word, and from the familiar ¶ style selection menu, Show Styles… again. Now, you have a word with a strike-through style applied.
If you want a red strikebar, choose the Color menu option, and pick red from the color chooser - before selecting Single from the strike-through menu. In the Fonts toolbar, there is a drop-down button offering single and double strike-through styles. While still in TextEdit, add and select an arbitrary word of default styled text. Click Add to Favorites, and name this Subscript. From the preceding ¶ style selector, select Show Styles… again. Reselect the "a" character, and now choose Format > Font > Baseline > Subscript. You want to return this to normal, so click the ¶ style selector on the toolbar again, and choose default. Leave the two checkboxes unselected, and click Add. A sheet will drop down allowing you to name this style, and I suggest you call it Superscript. Leave Document Styles selected, and simply click the Add to Favorites button. In the styles panel, you will see the superscripted letter "a" and below it, the current font info and the term Raised.
On the left end of the TextEdit toolbar, there is a ¶ selector button. Select the letter "a", and then choose Format > Font > Baseline > Superscript. Type the arbitrary text "ka" (without the quotes). Mail does not offer any Superscript, Subscript, or Strike-through style capability, but you can first create them in TextEdit, and then these styles will be available in Mail.