After MailEnable upgrade in Plesk 10.x mailbox disk usage is shown as '0 B' (zero bytes) at Domains > example.com > Control Panel > Mail tab
However, Statistics tab shows that mail disk usage is not zero.
Internal issue in MailEnable 6.03 that comes by default in Plesk 10.x. The issue has been fixed in MailEnable 6.5
Upgrade MailEnable 6.03 to 6.5 or later.
After upgrading MailEnable, go to Domains > example.com > Control Panel > Mail tab , and click on Refresh Usage Stats button.