Exchange 2010 - Receive connector conflict

I have a custom receive connector listening on port 25 which allows anonymous for my internal app servers. I'm aware the default connector also listens on port 25. Some application servers are failing to route emails and upon investigation they are hitting the default connector and bouncing with a 5.7.1.

The receive connector is listening on SMTP.domain.local and the apps are configured to send to that but some do and some don't. I'm sure in deployments in the past I have never had to setup the receive connector with a different port 2525 for example or an additional IP

Any ideas why this is happening?
StuartTechnical Architect - CloudAsked:
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Amit KumarCommented:
5.7.1 error is for either relay is access denied or client was not authenticated.

Please check if you added Application server's IP in connector?
StuartTechnical Architect - CloudAuthor Commented:
This was resolved by creating and additional IP and adding that to the receive connector, I then also removed the range from the default receive connector to not include the new IP

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