The following script will go through all your cPanel users and their domains, and print on the terminal a list of all current email accounts.
cat /etc/userdomains | sed "s/://g" | awk {'system("ls -1d /home/"$2"/mail/"$1"/* 2> /dev/null")'} | sed "s/\// /g" | awk {'print $5"@"$4'}