How to specify which IP address to use for an Exchange Send Connector
If you have multiple IP addresses on and Exchange 2010 Hub Transport server for the same network, Exchange will always use the preferred IP address for all outbound traffic and consequently, the send connector. There may be times when you want Exchange
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Read the complete post at http://www.expta.com/2011/05/how-to-specify-which-ip-address-to-use.html