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I do not recommend adding entire IP subnets that contain other Exchange servers as this can cause issues with server to server communications.

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If you want to provide a highly available SMTP service then a load balancer is the natural solution.

If you plan to load balance you'll need to ensure that the same receive connectors exist on all of the servers in the load balanced pool.

When authenticated SMTP is not an option you can create a new receive connector on the Exchange 2016 server that will allow anonymous SMTP relay from a specific list of IP addresses or IP ranges.

In the Exchange Admin Center navigate to mail flow and then receive connectors.

In my test environment that IP address will now be allowed to send email from any email address (whether it is a valid internal address or not) to any external address.