It seems that when I send a process too many non-blocking messages (with MPI_Isend) , MPI_Iprobe/MPI_Recv sometimes returns a buffer with corrupted data for some of the messages. Usually the corrupted data objects are at the end of the array that was sent. I checked the buffers passed to MPI_Isend, and they are uncorrupted. 1. Is there a way to detect this kind of overload with an MPI call? 2. Is there an upper bound on the number of messages that can be "in flight"? 3. Is there a upper bound on message length? Thanks, Kurt