http://blogs.technet.com/b/jribeiro/archive/2010/01/12/how-to-anonymously-relay-in-exchange-server-2007-2010.aspx

1. Create a new Receive Connector, name it "TEST" or whatever you want, and then select "Custom" for the intended use for the receive connector.
2. On the Local Network settings, leave it as is, because it will listen all local IP's on port 25.
3. On the Remote Network Settings, clear 0.0.0.0-255.255.255.255, and then add the IP Address of the remote server that requires relaying permissions.
4. Once the new Custom Receive Connector is created, go into the properties of this connector, go to the Permission Groups Tab and Add "Anonymous Users"
5. This is not everything. in order to activate the Anonymous users to relay via this connector, simply issue the following powershell cmdlet :

   Get-ReceiveConnector “TEST” | Add-ADPermission -User “NT AUTHORITY\ANONYMOUS LOGON” -ExtendedRights “Ms-Exch-SMTP-Accept-Any-Recipient”