Migrating a user from an IMAP Server to Zoho , during migration I was sent an email stating : 

since your mailbox size crossed the threshold limit

However when I went to the control panel to check Data usage the size still showed as 0 Gig’s

Fix

Goto the ‘Mail Storage‘ page in the Admin Control Panel and click on the ‘Refresh‘ icon which is alongside the user email address. This should update the used storage.