No you can use one public IP
domain.com MX = mail.domain.com
mail.domain.com A = IP
But if your giving your zimbra box a local IP, you're probably going to be interested in:
Split DNS - ZimbraWiki /blog/archives/2007/06/making_zimbra_bind_work_together_1.html
Well you'll probably eventually want more than 25 forwarded to that box:
Ports - ZimbraWiki
(you can change some of these to non standard ports as well)