This basically iterates through all the mailbox folders and counts the number of mail a particular mailbox has

megacounter3000

#!/bin/bash

for i in `zmprov -l gas | grep 'mbox'`; do
    for j in $( zmprov -l gaa -s ${i} | egrep -v "^(spam|ham)" ); do
        total=0
        echo -n "Total emails for $j = "
        for k in $( zmmailbox -z -m "$j" gaf | awk '{print $4}' | egrep -o "[0-9]+" ); do
            total=$(( total + k ))
        done
        echo $total
    done > /tmp/megacounteroutfile/${i}.outfile;
done