When you install Exchange 2007 it’s standard configured that it doesn’t allow extern mail.
Mostly you get the error “530 5.7.1 Client was not authenticated”
You can resolve this by giving the AnonymousUsers group receive rights.
You can do this via the exchange 2007 shell.
First get the receive connector name:
Then use the following command:
Set-ReceiveConnector -Identity “MAILSERVER\Default mailserver” -PermissionGroups “AnonymousUsers”