My client suspected some shenanigans by an employee so he asked me to search all accounts/inboxes for a certain string. Here's what I ran:

zmprov gaa |awk '{print "zmmailbox -z -m "$1" search <SEARCHSTRING>' |sh -v
This basically prints out each account then runs the search against each account. I figured someone else might be looking for a similar solution. Heck maybe there's an even easier way I overlooked.