iver is allowed to return an error from any block write call and the transport driver will call the data driver to delete what it had received so far and will discard any remaining blocks in this message as they are received, eventually returning an error to the sending transport driver). If this write fails (due to lack of disk space, for example), the receiving transport driver will return an error, which will be returned back to the Mailbox library on the sending machine. The user will be notified and th