“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: 550 550 5.7.1 Message rejected as spam by Content Filtering. (state 18).”
You can control the spam settings by opening the Exchange Management Console, go to the Organization Configuration, Hub Transport on the middle part you have a tab Anti Spam, here are the features to control the anti spam options.