The other day I was working on an issue that we could not send email outbound but inbound worked just fine. Looked to make sure we were not on any RBL. We are using Google Apps Web Email so it was not a 3rd Party client.
Delivery to the following recipient failed permanently:
The error that the other server returned was:
550-5.7.0 Mail relay denied [188.8.131.52]. Email is being sent from a domain
550-5.7.0 or IP address which isn’t registered in Google Apps account. Please
550-5.7.0 login to your Google Apps account and verify that your sending device
550-5.7.0 IP address has been registered within
550-5.7.0 Google Apps SMTP Relay Settings. For more information, please visit
550 5.7.0 https://support.google.com/a/
Postini is no longer working for you and you need to change the outbound gateway in Google Apps.
In Google Apps you need to change the mail routing.
Go to> Apps > Google Apps > Gmail > Advanced Settings: Then Find Outbound gateway