Hi Mikeyes,
Install two zimbra servers, one in internal network and one in DMZ in a way, share the LDAP service between these two servers (Zimbra Multi server installation). You can manage these two servers from a single admin console.
Read the multi server installation guide
http://www.zimbra.com/docs/ne/4.5.10...stall.5.1.html
Thanks,
#!Premod