CompuServe Messages

I've been reading some of the amiga system liturature available in the book stores and have a question about the message passing primatives. Does anyone know if they are "safe" multiprocessing primitives. For example if you were to set up a process as a shared data base handler, can the normal system routines handle situations where to different processes (tasks) send a message at the same time to the receiving process. I found the references to ports very confusing, how does the system keep track of who sent a message so the reply goes to the right task, are reply ports shared or does each sender have its own reply port? Can anyone reply with a reasonably terse explantion of how message passing and replying works?