The correct port for submission of external mail would be port 587, you can activate this in Zimbbra by doing the following:
and uncomment them, save the file and restart postfix. Make sure you leave the whitespace at the beginning of the lines. That port will require authentication and you should not have anyone trying to relay throiugh your system other then your external users.
In /opt/zimbra/postfix/conf/master.cf fine the following lines (near the beginning of the file):
# submission inet n - n - - smtpd
# -o smtpd_etrn_restrictions=reject
# -o smtpd_client_restrictions=permit_sasl_authenticated,reject