Overview

When a user exceeds his/her storage, an e-mail client may incorrectly report zero e-mails present in the Mailbox.

Reason

The mail server/Mail Delivery Agent - Dovecot, uses a  cache files to speed up mailbox access for E-Mail clients.
Running out of Disk-Quota/Space can interfere with proper operation of this mechanism.

Solution

• Login to the Control Panel
• Check User >  Manage Users to verify sufficient storage space is available (increasing limits if required)
• Navigate to Files > Filemanager and visit the Users Home Directory ( /home/<username> )
• Go to the Users Mailbox ( a folder named Mail/ ) 
• Delete the files: dovecot.index and dovecot.index.log and dovecot.index.cache