Hi Fahad,
By default, inbound mail originating from the internet will be received on the interface designated as the inbound mail IP address and this interface will then send the messages to the internal mail servers. In a similar fashion, outbound messages originating from the internal mail servers destined for the internet will be received on the interface designated as the outbound mail IP address and this interface will then send the message onwards.
In some situations, this may be undesirable behaviour and can be changed by editing a scanner from the Administration -> Hosts -> Configuration page, editing the scanner, clicking the SMTP tab, and then clicking the Advanced SMTP Settings button on the bottom of the page, check the option to ‘Reverse Address Binding Strategy’. After enabling the option to reverse the address binding strategy inbound mail originating from the internet will be received on the interface designated as the inbound mail IP address, but will be delivered to the internal mail servers from the interface designated as the outbound mail IP address. In a similar fashion, outbound messages originating from the internal mail servers destined for the internet will be received on the interface designated as the outbound mail IP address but will be delivered onwards from the interface designated as the inbound mail IP address.
Think that should sort you out.
Kevin