Thank God I did not have that problem! When I saw the extra mail box besides my wife's and mine, that’s what I did, deleted it, and all has been well since! Sorry I was not any help, if I was in your shoes I'd remove that Netscape folder (mine are in a Mozilla folder) and start over again, but keep the profile folder and move your mail boxes, bookmarks, and cookies back later, when you start Netscape you will have to go through the registration process again (I hit the cancel button), of course the bookmarks and such are gone, but quit the program and replace the things you saved from your old folder you should be up and running again, but I'd start with new pref's, in OSX there’s more you can move into the new profiles, what I've found fun is where they hide the user folder in 9 and below now, but what ever makes their day!
I don't use Netscape mail in my B/W, there I still use Eudora (what-the-hay it's what I'm use to), but on my 7500 I use it just to have the convenience of e-mail, but either way I like the tab feature of Netscape!