There may be times when you want Exchange to use another IP. Maybe because you already have that IP address configured in your firewalls. Unfortunately, you can't specify the IP address to use in Exchange for a send connector on Hub Transport servers, only Edge Transport servers.
You can, however, install a hotfix on Windows Server 2008 or Windows Server 2008 R2 servers which allows you to use netsh to disallow certain IPs to be used for outbound traffic. The hotfix is required for all versions of Windows Server 2008 SP2 and Windows Server 2008 R2 RTM. It is included in Windows Server 2008 R2 SP1.
Hotfix for Windows Server 2008 SP2: http://support.microsoft.com/kb/975808
Hotfix for Windows Server 2008 R2 RTM: http://support.microsoft.com/kb/2386184/
Once the hotfix is installed, run the following command from an elevated CMD prompt:
Netsh int ipv4 add address <Interface Name> <IP address> skipassource=trueThis will cause Windows to disallow the IP address on the specified NIC from being used for outbound network traffic.