No contents But Mail ID Showing 10MB Occupied

This could be due to 3 reasons.

1. Spam box,
2. The mails are left with a copy in the server
3. Webmail sent items and trash folders

Please clear the spam box, by clicking on spam assassin >> clear spam box option

For mails left with a copy in the server, please download them.

For webmail folders, login to webmail and delete them.