In the MPI Pro implementation, the standard, synchronous and buffered modes work as specified by the standard.

However, in the MPI Pro implementation, the ready mode always succeeds. In effect, the ready mode is equivalent to the standard send. The reason for this is that there is virtually no way of improving the performance based on knowing that the receiver has already posted the receive request by the time the send message arrives.

Therefore the only specific thing in MPI/Pro is that no matter how hard the programmer may try to cause an error using MPI_Rsend by making sure that the receive request is posted late, they will not succeed.