Run the below script as root, and after waiting you will be presented with the disk usage for each email account

CP_ACCOUNTS=`ls -1A /var/cpanel/users/`
for user in `echo -n $CP_ACCOUNTS`
do
domain=`grep -i ^dns /var/cpanel/users/$user |cut -d= -f2`
for dom in `echo -n "$domain"`
do
PASSWD_FILE="/home/$user/etc/$dom/passwd"
if [ -f $PASSWD_FILE ] && [ -s $PASSWD_FILE ]
then
for mail in `cat $PASSWD_FILE| cut -d":" -f1`
do
du -h --max=0 /home/$user/mail/$dom/$mail

done
fi
done
done