I depends on what you plan to deploy if you are using the open source solution or the network edition are you planning using only the web interface or are people going to be on POP and IMAP, if you are on the NE are you going to use the outlook connector.
In any case you'll need to do a multi-server install.
OS w/POP and IMAP
colo server with Zimbra MTA
local server with Zimbra Suite including another MTA, configure this MTA to relay to the external one. and configure clients with this MTA
OS just webmail
colo server with Zimbra MTA
local server with Zimbra Suite w/o MTA
NE -- with the Network Edition I'm assuming you'll use the Outlook Connector or Web Interface, there is no reason to use POP or IMAP with this
colo server with MTA
local server with Zimbra Suite
clients configured with Zimbra Outlook Connector |