When trying to send mail via Netscape, the message is displayed. The problem is that Netscape is trying to save the outbound message to a file that has been specified by the user, but does not exist.
To correct this problem: go to options Mail and News Preferences, then go to Compose. A template pops up. There is a section that specifies where to save out-going mail and news files. Make sure that these files exist or remove them from the template, if you do not care about logging what messages are sent via Netscape.