There is an issue with custom receive connectors on an Exchange server that host CAS and Mailbox roles. So an all in one Exchange server. The problem is with the New Custom Receive connector being associated with the HubTransport instead of the FrontendTransport. 

You can force the issue (replicate the problem) to occur faster if you shutdown the Microsoft Exchange Transport Service and Shutdown the Microsoft Exchange Frontend Transport Service… then start up the Microsoft Exchange Transport Service FIRST then start Frontend Service.  This will force the port 25 conflict that stops the external mail.

If you created a custom receive connector on Exchange and are experiencing issues delete that connector and recreate but this time choose Frontend instead of Hub for the role when creating it.

Shell command – New-Receiveconnector -Name <Name-It> -Usage Custom -Bindings -RemoteIpRanges <,etc> -TransportRole frontendtransport


OR the web interface



Hopefully this helps some of you fix your Exchange servers from not excepting external email.