How to create numbered sub-lists in Mail application?
In the Mail app go to the Format menu and click on lists and then on 'insert numbered list'. Go to your mail and enter text for the 1st item in the list. Now go back to the Format menu item and click on lists again and this time click on 'Increase List Level'. This will create sublists. See image below.



To go back to 1st level use Format/Lists/decrease list level.  You can create as many sublevels as you desire.