CRM 2015 eMail Router Issus

I have already tried both Exchange and SMTP settings for outbound eMail how ever a keep getting the following error. (NB THe eMail is Successfully sent, and because on this error it keep resenting ever 30min which is the default retry setting in the eMail router config file)

System.ServiceModel.FaultException`1[Microsoft.Xrm.Sdk.OrganizationServiceFault]: The entity cannot be updated because it is read-only. (Fault Detail is equal to Microsoft.Xrm.Sdk.OrganizationServiceFault)

This error is recorded in the CRM eMail Router Log file.

I presume that the eMail router is trying to update the eMail Activity in CRM to "sent", but does not have access to do so, as it is"Read Only" has anyone found this error and if so how is it fixed?

Thanks.