The official Microsoft documentation for Exchange Server is contradictory in terms of deploying an Exchange Server into your perimeter network (DMZ). In many cases, it is interpreted that placing an Exchange Server into this zone is a good idea.
This is a myth.
As a standard rule of managing your network, you should never place any machine joined to the domain into the DMZ. Exchange 2000, 2003 and 2007 (with the exception of the Edge Transport role - see below) must all be installed on machines joined to the domain - place them into the DMZ and you break the first rule of firewalls and Active Directory, which I mentioned above.
So why is this a bad idea?
An Exchange Server needs Active Directory to function because most of its configuration information is stored in the directory service. This is the reason why it must be deployed on a domain-joined server.
If you attempt to move an Exchange Server to the DMZ, you will quickly find that Exchange will break. This is because it loses the ability to find and communicate with the Domain Controllers on the private network. In situations like this, you would have to do one of two things:
Deploy an additional Domain Controller into the DMZ
Allow the Exchange Server access to the DCs on the private network
Completing either of the above tasks requires you to open ports between the DMZ and private network. The list of ports is extensive and includes sensitive services such as DNS, LDAP and NetBIOS. I heard a fellow Exchange Server MVP state the other day while referring to this list of ports: "open these ports and your firewall rules will look like Swiss Cheese"
The bottom line is this defeats the principle of a DMZ. A DMZ is intended as a "safe" location for machines which are not joined to the domain; you might put public web servers or public nameservers there, for example. In the DMZ, they are protected from the Internet, but anyone maliciously gaining access to those servers cannot cross the firewall into your private network. By opening the Active Directory ports I describe above and by placing a domain-joined machine in this insecure zone, any hacker in control of a compromised machine in the DMZ has a much easier route to access your Active Directory environment, perhaps bringing it to its knees.
Every Exchange MVP I know considers this to be a very, very bad idea. They would not configure an Exchange Server in this way and neither would I.
Any Exchange Server you deploy should always be on the private network. Located there, you can ensure it has access to the Domain Controllers without the need to compromise network security. From the outside, you only ever need ports 25 and 443 open to allow internal email to flow and for users to access Outlook Web Access and Exchange ActiveSync.
But what about Exchange 2000/2003 Front End Servers?
What about them? Again, it is a misconception - probably brought about by ambiguous documentation - that leads people to believe these servers are there for security reasons. They are not. Legacy Front-End Servers are designed for organisations with multiple mailbox servers. A front-end acts as a central connection point for access to OWA, OMA or ActiveSync under a single, common URL - it does not provide security.
If you are deploying a front-end server because you believe it will secure your Exchange environment, think again. Install Vamsoft ORF on a Virtual Machine or use an external spam filtering service as an alternative.
Exchange 2007 Edge Transport
With Exchange 2007, Microsoft has recognised this problem by adding the Edge Transport server role. This is the first time an Exchange Server role has been specifically designed to be located on the perimeter network. It is also the first time such a role exists for security reasons. The Edge Transport machine is designed to be on a workgroup - not a member of the domain - so it does not require sensitive ports to be opened between the DMZ and private network. It maintains its own copy of the Active Directory database using Active Directory Application Mode (ADAM) in Server 2003 or Active Directory Light-Weight Directory Services (AD LDS) in Server 2008.
I personally do not see a requirement for an Edge Transport server in an Exchange deployment, so I never deploy them. They are an unnecessary expenditure. Unlike a 2000/2003 front-end, they only process SMTP email traffic. Requests for OWA or Exchange Activesync still need to be made directly to the Client Access Servers (CAS), which are domain members and therefore still need to be located on the private network.
The minimal security advantage Edge Transport servers provide can easily be achieved directly on the Hub Transport servers - or by deploying a much cheaper Vamsoft ORF virtual server between the Internet and the Hub Transport server.
You should now have a better understanding of why an Exchange Server should not be deployed into the DMZ. I hope this prompts you to review your Exchange configuration and make appropriate changes to further improve your network security.