Hello Steve,
messages using CRES are encrypted and delivered on the Email Appliances (ESAs), where the secure envelope they generate contains the message as well. CRES only acts a a key storage server. Thus as there is no message content transfered to CRES, it's not archived anwhere. For the messages generated via theCRES web interface (i.e. secure reply), there is no way to archive them externally.
Hope that helps,
Andreas