EVERY time I launched Netscape v4.76, I'd get this message after the splash page ran:
You have configured Netscape Communicator to use an external mail program, but since
the POSTAL.DLL can't be found, Netscape messenger will be used instead.
I installed v4.78 in the hopes this would resolve the problem, to no avail.
I personally don't mind using Netscape messenger instead, since I don't recall ever trying
to configure Netscape to use an external mail program. But the error is a pain to deal with since it pops up every time and has to be manually cleared by clicking OK.