I have found on several Google Apps deployment there was not an SPF record set for the Google servers. So when a person was a member of a group and copied in an email on the Google Apps system, it would kick the message back.
Adding the required SPF record to the DNS server solved these issues and no more missed communications happened.
Most users never have a problem, but when they do it is important to quickly solve the issue because email is one of the most important forms of business communication.
If you are having issues with your Google Apps contact Lime Pi Digital at 1-866-599-0630.