1. A computer-implemented method for exchanging computer data in a data exchange between a first data processing system having a less restrictive data structure and a second data processing system having a more restrictive data structure, the method comprising: receiving from the first data processing system an indication of data to be deleted; determining whether a data structure associated with the second data processing system permits deleting the indicated data; and only sending the indication of data to be deleted received from the first data processing system to the second data processing system when the data structure associated with the second data processing system permits deleting the indicated data.