I've got three running behind some IPcop firewall/NAT's. I would strongly recommend putting it behind the firewall for two reasons...
1. The LAN clients will have first step access to it, eliminating network bottlenecks and stagger points.
2. While I have no doubt that a properly configured machine with decent passwords and IPtables can stand up for itself out on the big wide web, there is no such thing as too much security. It's obviously a stagger point but one worth having.
Forwarding the ports is not a huge job as depending on how you intend to use Zimbra there is between 1 and 8 ports to forward. |