GeoPud,
We can help to clarify further so that you may restore the desired contents from Time Machine.
When following the guide on restoring a particular app's files (see here: Restore items backed up with Time Machine on Mac ), it will note "On your Mac, open a window for the item you want to restore. For example, to recover a file you accidentally deleted from your Documents folder, open the Documents folder." So for emails to be restored from the Mail app, first open a Mail window.
Then, proceed with the same guide to launch Time Machine, and it should show the different 'ticks' on the right side, representing different backups. Select the desired date/backup in here, and then you may restore the content as needed. Restored items will then return to their original location.
We hope that helps out a bit.