The follwing error message reports an corrupt mail:
Archive Process spooler error: An exception was raised on the server: AddMailTransactionFile Exception: [/mnt/storage/...] - Mail Transaction Error. There is not Action with index (1) stored
This corrupt transaction seldomly happens and is mainly caused by system failure (PowerOff / Reset / Powerfailure).
This transaction cannot be fixed.
It only can be deleted via Admin Webinterface -> MailDepot -> Archive Spooler -> right click -> delete.