CAS Server in perimeter

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We currently have a few FE 2003 Exchange servers in the perimeter
network. We are planning replace those with 2007 CAS servers.

I understand that Microsoft does not officially support CAS servers in
the perimeter. However, I know people are doing it successfully. We
do not currently have the funds for any other servers so I need to do
this.

Are there any additional firewall holes that CAS servers require? Are
there any special configuration changes I need to be aware of?
Gotchas?

I appreciate your help.

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