Mailbox size in Plesk UI differs from the actual size on disk

Symptoms

• Mail users receive Mailbox usage exceeded mail notifications that indicate they have gone over the quota limits, even though the mailbox directory size indicates that they have not done so
  
• Mailbox shows the incorrect size in Plesk UI that differs from the size of the directory /var/qmail/mailnames/example.com/jdoe
• The file /var/qmail/mailnames/example.com/jdoe/Maildir/maildirsize contains wrong data about the size of the mailbox
• Mailbox directory/var/qmail/mailnames/example.com/jdoe/Maildir contains one or several additional folders starting with .INBOX

Cause

Invalid mailbox subfolders.  A mail client that is used to manage the mailbox creates several ".INBOX" (for example .INBOX.Drafts .INBOX.Sent .INBOX.Trash) directories, which results in Dovecot calculating mail disk usage more than once and it therefore creates an inaccurate maildirsize.

This Dovecot bug with ID DOP-1336 is recognized officially in the following official thread:
subject:"Re\: \[BUG\?\] Double quota calculation when special folder is present"

Resolution

Rename the .INBOX folders in the used mail client to folders that would not start with .INBOX in order to resolve this issue.