However, there are several factors that complicate the answer to this question. Many email systems, especially Microsoft Exchange, make it quite difficult to separate the webmail front end from the email back end. They require punching so many holes in the firewall -- to allow communication between the two systems -- that they limit the effectiveness of placing them in different network zones.
If you have some flexibility in your network topology, one potential workaround is to create a separate email network zone that is firewalled from both the DMZ and your internal network, and then place both the email and webmail servers in that zone. You may then allow client access there over traditional "fat client" ports from the internal network and webmail ports from the Internet.
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