IP-based access control lists (ACLs)
The appropriate IP address(es) of your SMTP forwarder(s) must be provided during the SmartRelay setup process. Further IP addresses can be added later, if required.
Because your assigned SMTP listener is dedicated to only accepting incoming SMTP connections from those pre-authorized IP address(es), you remain in full control of implementing your own security policies to enforce authorized access. The only way to access your SmartRelay implementation is through your own environment.
See SmartRelay overview for more about the SMTP listener and the local SMTP forwarder.