If you go over quota in webmail and cannot delete messages and are unable to send or receive mail.
Logon to webmail, click the Options button.
Go to Mail Management, click deleting and moving messages.
Uncheck the box When deleting messages, move them to your Trash folder instead of marking them as deleted? Next click Save Options.
You will now see an icon in the right hand side under Move | Copy named Show Deleted | Purge Deleted; after you delete your mail, click Purge Deleted, this will permanently delete your messages.