Hi @Luke Kenny
Glad to know that your issue is resolved now, and thanks for sharing the solution! I would make a brief summary of this post so that other forum members could easily find useful information here:
[Exchange 2016 using wrong network interface - Summary]
Issue Symptom:
A send connecter is created to send out emails via a Smart Host. While adding the Received header of the network interface, it shows random.
Solution:
Specify the correct network interface using the parameter SourceIPAddress
The SourceIPAddress parameter specifies the local IP address to use as the endpoint for an SMTP connection to a remote messaging server. The default IP address is 0.0.0.0. This value means that the server can use any available local IP address. This parameter is valid only for Send connectors configured on an Edge server.
Reference Links:
Set-SendConnector
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