Email Space usage too much
Please check: 


If you start the Mail.app, you get a list of Shortcuts (in the upper
left corner)
Below that you should see a list of your accounts. If
you click on one of your accounts, you get a list of all your
Folders/Mailboxes (i hope i translated it correctly). 
Check if the Trash-Folder is empty or if there are still Mails marked for
deletion. 
Repeat that for all your Mailaccounts and check other Folders like the  'Draft'-Folder too.


Alternative Option: If you have all Mail-Account Credentials, delete all Accounts and re-create them. Perhaps that will change the Usage. Please check before re-creating them.