Please note: Google has deprecated C2DM and replaced the service with GCM. C2DM continues to work, though auth tokens can still randomly expire. To migrate your application to GCM, please see our getting started docs here.
If you see the following error message in the log at go.urbanairship.com:
CRITICAL ERROR Quota exceeded
This means that the C2DM quota allocated to you has been exceeded. Since Google has deprecated C2DM, they are no longer accepting requests to increase your daily push quota.