You are unable to relay with an SMTP error 5.7.57 Client was not authenticated.

You have configured an anonymous relay connector on your Exchange server and entered the source IP address of the sending system. However the client is redirected to the default connector which tries to authenticate the sender and throws error 5.7.57 Client was not authenticated.

This can happen if you have multiple IP addresses on the sending host and it is sending from an address other than the one you entered in the relay connector configuration.

There is a fairly easy way to determine the sending IP address:

  1. From the sending system open the Telnet client and try to connect to your Exchange server on port 25.
  2. Type ehlo. 
  3. The Exchange server will greet you, mentioning the IP address the connection is originating from.

Dejan Foro