If Netscape mail gives you an error of the form "can't open FCC file",
then Netscape is trying to save a copy of your outgoing mail to a file
that does not exist.  To fix this, you should choose "Mail & News
Preferences" from the "Options" menu, then click on the "Compose" tab.
 
Next, make sure the "Mail Messages:" boxes are empty (so Netscape doesn't
try to mail or save a copy of each message).  Finally, click on "OK"
to save these options.