On your cPanel dedicated server, for an email the mailbox quota is displayed wrong ,
Go through the following steps:
Login to your server as root
cd /home/username/mail/domain.com/emailaccount
rm -rf maildirsize
Now, login to cPanel
Go to email account and change the quota for particular email account.
(Once you change quota, it will recreate file maildirsize automatically)
You can also delete maildirsize file from cpanel > file manager
