You should can disable port 25 on zimbra. But u also need to configure another box which listens on 25 and forward emails to zimbra on modified port. I dont think that makes much difference.
Another solution you can try is to configure submission on zimbra postfix, which makes portfix listen on two ports i.e. 587 and 25. And set several restrictions on 587. Many ISPs lock down 25 outbound so their customers can't connect to port 25 on anybody else's servers. I am not sure if you are looking for this.
Racker Hacker » Enable submission port 587 in Postfix