When mails are being sent to my exchange 2007 server (from gmail) they are being returned with the message
Technical details of permanent failure: Google tried to deliver your message, but it was rejected by the recipient domain. We recommend contacting the other email provider for further information about the cause of this error. The error that the other server returned was: 530 530 5.7.1 Client was not authenticated (state 13).
What can be causing this?