Ric Ford reported on ZiffNet/Mac that Navigator has recently (since about the end of August) stopped replying correctly to messages from the Internet. The problem, when we checked and as Mike O'Connor, Navigator's author, confirmed, stems from CompuServe not including the initial ">" at the beginning of the Internet address. Without that character prefixing the address (see TidBITS-141 for the gory details of Internet/CompuServe connections), the mail never gets out of CompuServe.
Both the blame and the responsibility for a fix lie with CompuServe because as Ric said, Navigator hasn't suddenly changed overnight. In addition, in my tests CIM 2.0.1 seems to suffer the same problem, although I didn't check a normal terminal session. The only workaround in the meantime is creating a new message with the proper address format. Sorry about that, folks.