I've often quoted Scott Landry's post on the team blog— SMTP Virtual Server Myths Exposed. Myth #4 in Scott's post:
Myth 4: Virtual Server IP Address Will Be Used For Outgoing ConnectionsExchange Server 2007, with its shiny new transport stack (freshly divorced from IIS' SMTP service), makes this quite clear. Receive Connectors, somewhat comparable to the SMTP Virtual Server in previous versions, are for receiving inbound mail. Send Connectors are for sending outbound mail.
The last source of misunderstanding is the socket which will be used to open an SMTP connection. This may seem confusing and somewhat contradictory of my first point, but SMTP simply tells the Windows network stack to provide SMTP with a socket. It does not provide a source IP address to use, and as such, you will notice that the source IP address assigned by Windows will be based on the Windows routing table, not taking into consideration the IP of the SMTP VSI that is delivering the message. A common observation of this is that on a cluster server we are using the physical machine IP as our source IP, not any of the virtual IP addresses.
When creating or modifying a Send Connector using the shell, you can specify the SourceIPAddress parameter to configure it to use a particular IP address for outbound mail. The IP address can be any IP address bound to a NIC on the Edge Transport server that is configured as a source server on the Send Connector. To modify an existing Send Connector, using the following command:
Set-SendConnector "ToInternet" -SourceIPAddress 18.104.22.168However, as noted in the documentation, this only works on Edge Transport servers. Hub Transport servers ignore the SourceIPAddress parameter.