[uxbod]

#!/bin/bash

echo "Account,MailQuota,QuotaUsed"
zmprov gaa {whateverdomain} | while read ACCOUNT
do
    QUOTA=`zmprov ga ${ACCOUNT} | grep "zimbraMailQuota" | awk -F\: '{ print $2 }'`
    USAGE=`zmmailbox -z -m ${ACCOUNT} gms`
    echo "${ACCOUNT},${QUOTA},${USAGE}"
done > accounts.csv

Job done