[carlsanders]

OK, jumping in late here.

I run a split dns and split e-mail. My external e-mail is on a DMZ. DNS has:

mailserver.domain.com, with an external IP.

My firewall does the NAT from outside to DMZ. I forward e-mail from my DMZ
e-mail server to my internal, zimbra e-mail server. My external DNS has
zimbra.domain.com. My firewall takes https requests and sends them to my
internal zimbra server with a NAT'd IP.

My internal dns has:

internalname.domain.com, with an internal IP.

I just added a cname record to my internal dns:

mailserver in cname internalname

So, while external users are reaching my zimbra server via an external IP,
the domain name is the same as my internal users: zimbra.domain.com

Hope this helps. I find the split e-mail solution to be quite elegant. I use
postfix, and it allows me to forward to any given IP of an e-mail server based on
the domain in question.

PS Zimbra's admin section allows one to change the IP of the outgoing mail
server. Perhaps this will help...