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

• Mailbox directory/var/qmail/mailnames/example.com/jdoe/Maildir does not contain any additional folders starting with .INBOX But through the utility doveadm you can see duplicate folders:

  # doveadm mailbox list -u jdoe@example.com
  INBOX
  INBOX.INBOX
  INBOX.INBOX.Sent
  INBOX.Drafts
  INBOX.Deleted Messages
  INBOX.Sent
  INBOX.Trash

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. You also need to delete duplicate folders starting with INBOX.

Note: Make a backup of your mailbox before making any changes.

You can also use the doveadm utility on the server to move emails and delete duplicates:

# doveadm -v move -u jdoe@example.com INBOX.Sent mailbox INBOX.INBOX.Sent ALL
# doveadm -v move -u jdoe@example.com INBOX mailbox INBOX.INBOX ALL
# doveadm mailbox delete -u jdoe@example.com -sre INBOX.INBOX.Sent
# doveadm mailbox delete -u jdoe@example.com -sre INBOX.INBOX
# doveadm mailbox list -u jdoe@example.com
# doveadm quota get -u jdoe@example.com