I had this issue on a clients site recently, when a user used OWA and tried to read an email they had already sent in Outlook Web Access they would see the error:
Additional to this, in OWA when the user tried to compose a new email then send the email they are greeted with the following error:
To give a brief overview of my Exchange 2013 environment I had 3 servers one with the CAS role involved and 2 with the MBX role installed in a DAG, these servers where all on Server 2012 R2 Data centre. The issue however may also effect a single Exchange 2013 server with all roles installed.
The issue seems to be that during the installation of Exchange 2013 the permissions on the Exchange Back end are not set correctly. To make sure we get this right, in your particular environment, any server with the MBX role installed, in IIS the OWA virtual directory must have anonymous permissions allow as seen below.
Once these permissions are set on the MBX role servers run:
iisreset
Then log in back into OWA, all emails will now display correctly.