How to empty all emails data when account deleted?
Assuming you have deleted the Gmail and Outlook accounts, if you preform a restore using iTunes and backups, all the email data will be free and so return available. 

EDIT: If you want you can also try with some specific application (e.g., PhoneClean) that help to clean the phone. Anyway, before download third part app I think the restore procedure (that is free) can work as well.