System Restore only affects your PC's registry. It has nothing to do with individual data files, such as emails.
Email clients (such as Windows Mail) don't store emails as individual files. What appear to be (and are referred to as) 'folders' (such as 'Inbox' and 'Deleted Mail') are actually single data files, which are rewritten whenever you move, or delete, an email. (Think of it as similar to having a single Word file, containing a book made up of lots of chapters. If you delete just one chapter, you end up with a modified version of the same 'book' file. There was never a file which contained just that one chapter. Similarly there was never a file which contained the important email you're seeking). That method of working makes it virtually impossible to recover a deleted email, since it never existed as a separate file in its own right.
I have very occasionally managed to find very limited data from a deleted email (such as important phone number) by using the following technique, but it definitely won't find things like embedded images or attachments:
Run a search for 'Deleted Mail' (or 'Trash', or whatever else your deleted mail folder is called). When found, right-click on it and select 'Open With'. Choose 'Notepad'. That will show you everything which remains in that folder, which MIGHT still include elements of your deleted mail.
Chris