[uxbod]

Something along these lines would do what you want.

su - zimbra

#!/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