If a user has quota issues with a mail account, or cannot change the quota in cPanel, you (or the user) can delete the maildirsize file in their mail account files:

Using the cPanel File Manager, or in shell (SSH) from the account homedir:

rm -f mail/*/maildirsize
rm -f mail/*/*/maildirsize

The above commands will remove the maildirsize files for the cPanel account and the domain mail accounts.  The disk usage for mail accounts may take up to 4 hour to display correctly in cPanel (for cPanel disk calculations).

On some server, your must run “/scripts/fixquotas”.  If you are still having, you may have to restart mail services (courier-imap, courier-authlib, exim) and run “/scripts/mailperm”.  To force cPanel disk calculation, you can restart cPanel (service cpanel restart).